Method and apparatus for handover without interruption of transmission and reception of data in next-generation mobile communication system

ABSTRACT

The present disclosure relates to a communication method and system for converging a 5th-Generation (5G) communication system for supporting higher data rates beyond a 4th-Generation (4G) system with a technology for Internet of Things (IoT). The present disclosure may be applied to intelligent services based on the 5G communication technology and the IoT-related technology, such as smart home, smart building, smart city, smart car, connected car, health care, digital education, smart retail, security and safety services. A method of a terminal in a wireless communication system according to the disclosure includes receiving, from a first base station, a message including a command of handover from a first base station to a second base station through radio resource control (RRC) signaling; identifying the message includes information indicating dual protocol stack handover; and receiving data until a cell of the first base station is released.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of application Ser. No. 17/670,327,filed Feb. 11, 2022, which is a continuation of application Ser. No.16/834,922, filed Mar. 30, 2020, now U.S. Pat. No. 11,252,624, which isbased on and claims priority under 35 U.S.C. § 119(a) of a Korean patentapplication number 10-2019-0035805, filed on Mar. 28, 2019, in theKorean Intellectual Property Office, and of a Korean patent applicationnumber 10-2019-0046156, filed on Apr. 19, 2019, in the KoreanIntellectual Property Office, and of a Korean patent application number10-2019-0115518, filed on Sep. 19, 2019, in the Korean IntellectualProperty Office, the disclosures of which are incorporated by referenceherein in their entirety.

BACKGROUND 1. Field

The disclosure relates to a method and an apparatus for efficienthandover without interruption of transmission and reception of dataduring handover in a next-generation mobile communication system.

2. Description of Related Art

To meet the demand for wireless data traffic having increased sincedeployment of 4G communication systems, efforts have been made todevelop an improved 5G or pre-5G communication system. Therefore, the 5Gor pre-5G communication system is also called a “Beyond 4G Network” or a“Post LTE System”. The 5G communication system is considered to beimplemented in higher frequency (mmWave) bands, e.g., 60 GHz bands, soas to accomplish higher data rates. To decrease propagation loss of theradio waves and increase the transmission distance, the beamforming,massive multiple-input multiple-output (MIMO), full dimensional MIMO(FD-MIMO), array antenna, an analog beam forming, large scale antennatechniques are discussed in 5G communication systems. In addition, in 5Gcommunication systems, development for system network improvement isunder way based on advanced small cells, cloud radio access networks(RANs), ultra-dense networks, device-to-device (D2D) communication,wireless backhaul, moving network, cooperative communication,coordinated multi-points (CoMP), reception-end interference cancellationand the like. In the 5G system, hybrid FSK and QAM modulation (FQAM) andsliding window superposition coding (SWSC) as an advanced codingmodulation (ACM), and filter bank multi carrier (FBMC), non-orthogonalmultiple access (NOMA), and sparse code multiple access (SCMA) as anadvanced access technology have also been developed.

The Internet, which is a human centered connectivity network wherehumans generate and consume information, is now evolving to the Internetof things (IoT) where distributed entities, such as things, exchange andprocess information without human intervention. The Internet ofeverything (IoE), which is a combination of the IoT technology and thebig data processing technology through connection with a cloud server,has emerged. As technology elements, such as “sensing technology”,“wired/wireless communication and network infrastructure”, “serviceinterface technology”, and “security technology” have been demanded forIoT implementation, a sensor network, a machine-to-machine (M2M)communication, machine type communication (MTC), and so forth have beenrecently researched. Such an IoT environment may provide intelligentInternet technology services that create a new value to human life bycollecting and analyzing data generated among connected things. IoT maybe applied to a variety of fields including smart home, smart building,smart city, smart car or connected cars, smart grid, health care, smartappliances and advanced medical services through convergence andcombination between existing information technology (IT) and variousindustrial applications.

In line with this, various attempts have been made to apply 5Gcommunication systems to IoT networks. For example, technologies such asa sensor network, machine type communication (MTC), andmachine-to-machine (M2M) communication may be implemented bybeamforming, MIMO, and array antennas. Application of a cloud radioaccess network (RAN) as the above-described big data processingtechnology may also be considered an example of convergence of the 5Gtechnology with the IoT technology.

The above information is presented as background information only toassist with an understanding of the disclosure. No determination hasbeen made, and no assertion is made, as to whether any of the abovemight be applicable as prior art with regard to the disclosure.

SUMMARY

In the next-generation mobile communication system, a method forefficient handover is required to support a seamless data service withlow transmission latency.

In order to solve the above problem, the disclosure provides a method ofa terminal in a wireless communication system, which may include:receiving, from a first base station, a message including a command ofhandover from a first base station to a second base station throughradio resource control (RRC) signaling; identifying whether the messageincludes information indicating dual protocol stack handover; andreceiving, from the first base station, data until a cell of the firstbase station is released in case that the message includes informationindicating dual protocol stack handover.

In addition, in order to solve the above problem, the disclosureprovides a method of a first base station in a wireless communicationsystem, which may include: transmitting, to a terminal, a messageincluding a handover command to a second base station through radioresource control (RRC) signaling; and transmitting, to the terminal,data until a cell of the first base station is released in case that themessage includes information indicating dual protocol stack handover.

In addition, in order to solve the above problem, the disclosureprovides a terminal in a wireless communication system, which mayinclude: a transceiver; and a controller configured to receive, from afirst base station via the transceiver, a message including a command ofhandover from a first base station to a second base station throughradio resource control (RRC) signaling using the transceiver; identifywhether the message includes information indicating dual protocol stackhandover; and receive, from the first base station via the transceiver,data until a cell of the first base station is released in case that themessage includes information indicating dual protocol stack handover.

Further, in order to solve the above problem, the disclosure provides afirst base station in a wireless communication system, which mayinclude: a transceiver; and a controller configured to transmit, to aterminal via the transceiver, a message including a handover command toa second base station through radio resource control (RRC) signaling,and, transmit, to the terminal via the transceiver, data until a cell ofthe first base station is released in case that the message includesinformation indicating dual protocol stack handover.

The disclosure proposes various methods for efficient handover in orderto prevent the occurrence of data interruption time due to handover whenperforming handover in a next-generation mobile communication system,thereby supporting seamless data services.

Before undertaking the DETAILED DESCRIPTION below, it may beadvantageous to set forth definitions of certain words and phrases usedthroughout this patent document: the terms “include” and “comprise,” aswell as derivatives thereof, mean inclusion without limitation; the term“or,” is inclusive, meaning and/or; the phrases “associated with” and“associated therewith,” as well as derivatives thereof, may mean toinclude, be included within, interconnect with, contain, be containedwithin, connect to or with, couple to or with, be communicable with,cooperate with, interleave, juxtapose, be proximate to, be bound to orwith, have, have a property of, or the like; and the term “controller”means any device, system or part thereof that controls at least oneoperation, such a device may be implemented in hardware, firmware orsoftware, or some combination of at least two of the same. It should benoted that the functionality associated with any particular controllermay be centralized or distributed, whether locally or remotely.

Moreover, various functions described below can be implemented orsupported by one or more computer programs, each of which is formed fromcomputer readable program code and embodied in a computer readablemedium. The terms “application” and “program” refer to one or morecomputer programs, software components, sets of instructions,procedures, functions, objects, classes, instances, related data, or aportion thereof adapted for implementation in a suitable computerreadable program code. The phrase “computer readable program code”includes any type of computer code, including source code, object code,and executable code. The phrase “computer readable medium” includes anytype of medium capable of being accessed by a computer, such as readonly memory (ROM), random access memory (RAM), a hard disk drive, acompact disc (CD), a digital video disc (DVD), or any other type ofmemory. A “non-transitory” computer readable medium excludes wired,wireless, optical, or other communication links that transporttransitory electrical or other signals. A non-transitory computerreadable medium includes media where data can be permanently stored andmedia where data can be stored and later overwritten, such as arewritable optical disc or an erasable memory device.

Definitions for certain words and phrases are provided throughout thispatent document. Those of ordinary skill in the art should understandthat in many, if not most instances, such definitions apply to prior, aswell as future uses of such defined words and phrases.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the disclosure and its advantages,reference is now made to the following description taken in conjunctionwith the accompanying drawings, in which like reference numeralsrepresent like parts:

FIG. 1 is a diagram illustrating the structure of an LTE system to whichthe disclosure may be applied;

FIG. 2 is a diagram illustrating a radio protocol structure of an LTEsystem to which the disclosure may be applied;

FIG. 3 is a diagram illustrating the structure of a next-generationmobile communication system to which the disclosure may be applied;

FIG. 4 is a diagram illustrating a radio protocol structure of anext-generation mobile communication system to which the disclosure maybe applied;

FIG. 5 is a diagram illustrating the procedure in which a terminalswitches from an RRC idle mode to an RRC connected mode and establishesa connection with a network in the disclosure;

FIG. 6 is a diagram illustrating signaling procedures for performinghandover in a next-generation mobile communication system;

FIG. 7 is a diagram illustrating a first embodiment of an efficienthandover method for minimizing data interruption time due to handover inthe disclosure;

FIG. 8 is a diagram illustrating second and third embodiments of anefficient handover method for minimizing data interruption time due tohandover in the disclosure;

FIG. 9 is a diagram illustrating a fourth embodiment of an efficienthandover method for minimizing data interruption time due to handover inthe disclosure;

FIG. 10 is a diagram illustrating a fifth embodiment of an efficienthandover method for minimizing data interruption time due to handover inthe disclosure;

FIG. 11 is a diagram illustrating the structure of an efficient PDCPentity that may be applied to the embodiments;

FIG. 12 is a diagram illustrating the operation of a terminal that maybe applied to the embodiments proposed in the disclosure;

FIG. 13 illustrates the structure of a terminal to which an embodimentmay be applied; and

FIG. 14 illustrates a block diagram of a TRP in a wireless communicationsystem to which an embodiment may be applied.

DETAILED DESCRIPTION

FIGS. 1 through 14 , discussed below, and the various embodiments usedto describe the principles of the present disclosure in this patentdocument are by way of illustration only and should not be construed inany way to limit the scope of the disclosure. Those skilled in the artwill understand that the principles of the present disclosure may beimplemented in any suitably arranged system or device.

Hereinafter, the operation principle of the disclosure will be describedin detail in conjunction with the accompanying drawings. In thefollowing description of the disclosure, a detailed description of knownfunctions or configurations incorporated herein will be omitted when itmay make the subject matter of the disclosure rather unclear. The termswhich will be described below are terms defined in consideration of thefunctions in the disclosure, and may be different according to users,intentions of the users, or customs. Therefore, the definitions of theterms should be made based on the contents throughout the specification.

In the following description of the disclosure, a detailed descriptionof known functions or configurations incorporated herein will be omittedwhen it may make the subject matter of the disclosure rather unclear.Hereinafter, embodiments of the disclosure will be described withreference to the accompanying drawings.

In the following description, terms for identifying access nodes, termsreferring to network entities, terms referring to messages, termsreferring to interfaces between network entities, terms referring tovarious identification information, and the like are illustratively usedfor the sake of convenience. Therefore, the disclosure is not limited bythe terms as used below, and other terms referring to subjects havingequivalent technical meanings may be used.

In the following description, the disclosure uses terms and namesdefined in 3rd generation partnership project long term evolution (3GPPLTE) standards for the convenience of description. However, thedisclosure is not limited by these terms and names, and may be appliedin the same way to systems that conform other standards. In thedisclosure, the term “eNB” may be interchangeably used with the term“gNB” for the convenience of description. That is, a base stationdescribed as “eNB” may refer to “gNB”.

The disclosure proposes seamless handover methods capable of minimizingdata interruption time due to handover or reducing the same to 0milliseconds (ms) in a next-generation mobile communication system.

Specifically, efficient handover methods proposed in the disclosure mayhave one or more of a plurality of features as follows.

-   -   If a terminal that transmits and receives data to and from a        source base station (uplink or downlink data        transmission/reception) through respective protocol entities of        a plurality of first bearers (PHY entities, MAC entities, RLC        entities, or PDCP entities) receives a handover command message        (or an RRC reconfiguration message) from the source base        station, the terminal may configure new protocol entities of a        plurality of second bearers corresponding to (for example,        having the same bearer identifiers as) the protocol entities of        the plurality of first bearers, and may maintain the        transmission and reception of data (uplink or downlink data        transmission/reception) to and from the source base station        (without stopping the same) through the plurality of first        bearers, thereby performing the transmission and reception of        data (uplink or downlink data transmission/reception).    -   The protocol entities of the plurality of second bearers (PHY        entities, MAC entities, RLC entities, or PDCP entities), which        are newly configured after receiving the handover command        message as described above, may be configured for transmitting        and receiving data to and from a target base station, based on        bearer configuration information or protocol entity information        included in the handover command message.    -   The terminal may perform a procedure of random access to a        target base station through protocol entities of the plurality        of second bearers (for example, MAC entities) while transmitting        and receiving data to and from the source base station (uplink        or downlink data transmission/reception) through the protocol        entities of the plurality of first bearers. In this case, the        random access procedure may include transmitting a preamble,        receiving a random access response, or transmitting message 3.    -   The terminal may complete a procedure of random access to the        target base station through the protocol entities of the        plurality of second bearers (for example, MAC entities) while        transmitting and receiving data to and from the source base        station (uplink and downlink data transmission and reception)        through the protocol entities of the plurality of first bearers,        and may transmit a handover completion message to the target        base station through the protocol entities of the plurality of        second bearers.    -   The terminal may complete a procedure of random access to the        target base station through the protocol entities of the        plurality of second bearers (for example, MAC entities) while        transmitting and receiving data to and from the source base        station (uplink and downlink data transmission and reception)        through protocol entities of the plurality of first bearers, may        transmit a handover completion message to the target base        station through the protocol entities of the plurality of second        bearers, and may perform (uplink or downlink) transmission and        reception of data.    -   The terminal may stop transmitting and receiving data to and        from the source base station through the protocol entities of        the plurality of first bearers when a procedure of random access        to the target base station is completed {for example, when a        random access response is received, when a handover completion        message (e.g., an “RRCReconfiguration” message) is transmitted        to the target base station, or when data is initially        transmitted to the target base station using a PUCCH or PUSCH        uplink transmission resource}.    -   When a handover command message is received, the terminal may        continue to transmit and receive data to and from the source        base station (uplink or downlink data transmission/reception)        through the protocol entities of the plurality of first bearers,        and may perform a procedure of random access to the target base        station through the protocol entities of the plurality of second        bearers, and when a random access response is received, when a        handover completion message is transmitted to the target base        station, or when data is initially transmitted using a PUCCH or        PUSCH uplink transmission resource, the terminal may stop        transmitting uplink data to the source base station through the        protocol entities of the plurality of first bearers, and may        transmit uplink data to the target base station only through the        protocol entities of the plurality of second bearers, and the        terminal may continue to receive downlink data from the source        base station through the protocol entities of the plurality of        first bearers, and may continue to receive downlink data from        the target base station through the protocol entities of the        plurality of second bearers.

Hereinafter, the disclosure proposes efficient handover procedureswithout data interruption time, based on the above features.

FIG. 1 is a diagram illustrating the structure of an LTE system to whichthe disclosure may be applied.

Referring to FIG. 1 , a radio access network of an LTE system mayinclude evolved Node Bs (hereinafter, referred to as “ENBs”, “Node Bs”,or “base stations”) 105, 110, 115, and 120, a mobility management entity(MME) 125, and a serving-gateway (S-GW) 130, as shown in FIG. 1 . Userequipment (hereinafter, referred to as “UE” or a “terminal”) 135accesses an external network through the ENBs 105 to 120 and the S-GW130.

In FIG. 1 , the ENBs 105 to 120 may correspond to existing Node Bs of aUNITS system. The ENB may be connected to the UE 135 via a radiochannel, and may play a more complex role than the existing Node B. Inthe LTE system, all user traffic including real-time services, such asvoice-over-IP (VoIP) through the Internet protocol, is served through ashared channel. Therefore, a device for collecting status information,such as buffer status, available transmission power status, and channelstatus of UEs, and performing scheduling is used. The ENBs 105 to 120serve as such a device. One ENB typically controls multiple cells. Forexample, in order to realize a data rate of 100 Mbps, the LTE systemuses, as radio access technology, orthogonal frequency divisionmultiplexing (hereinafter, referred to as “OFDM”) in, for example, a 20MHz bandwidth. In addition, an adaptive modulation and coding(hereinafter, referred to as “AMC”) scheme is applied to determine amodulation scheme and a channel coding rate in accordance with thechannel status of a terminal. The S-GW 130 is a device for providingdata bearers, and generates or removes data bearers under the control ofthe MIME 125. The MIME is a device that performs various controlfunctions, as well as a mobility management function for a terminal, andmay be connected to a plurality of base stations.

FIG. 2 is a diagram illustrating a radio protocol structure of an LTEsystem to which the disclosure may be applied.

Referring to FIG. 2 , the radio protocol of an LTE system includespacket data convergence protocol (PDCP) 205 or 240, radio link control(RLC) 210 or 235, and medium access control (MAC) 215 or 230 in aterminal and an ENB, respectively. The packet data convergence protocol(PDCP) 205 or 240 performs operations, such as IP headercompression/decompression and the like. The primary functions of thePDCP are summarized as follows.

-   -   Header compression and decompression (ROHC only)    -   Transfer of user data    -   In-sequence delivery of upper layer PDUs at PDCP        re-establishment procedure for RLC AM    -   Sequence reordering {for split bearers in DC (only support for        RLC AM): PDCP PDU routing for transmission and PDCP PDU        reordering for reception}    -   Duplicate detection of lower layer SDUs at PDCP re-establishment        procedure for RLC AM    -   Retransmission of PDCP SDUs at handover and, for split bearers        in DC, of PDCP PDUs at PDCP data-recovery procedure, for RLC AM    -   Ciphering and deciphering    -   Timer-based SDU discard in uplink.

The radio link control (RLC) 210 or 235 reconfigures a PDCP PDU (packetdata unit) to an appropriate size and performs ARQ operation and thelike. The primary functions of the RLC are summarized as follows.

-   -   Data transfer function (transfer of upper layer PDUs)    -   ARQ function {error correction through ARQ (only for AM data        transfer)}    -   Concatenation, segmentation, and reassembly of RLC SDUs (only        for UM and AM data transfer)    -   Re-segmentation of RLC data PDUs (only for AM data transfer)    -   Reordering of RLC data PDUs (only for UM and AM data transfer)    -   Duplicate detection (only for UM and AM data transfer)    -   Protocol error detection (only for AM data transfer)    -   RLC SDU discard (only for UM and AM data transfer)    -   RLC re-establishment

The MAC 215 or 230 is connected to a plurality of RLC entitiesconfigured in a terminal, multiplexes RLC PDUs into MAC PDUs, anddemultiplexes RLC PDUs from MAC PDUs. The primary functions of the MACare summarized as follows.

-   -   Mapping between logical channels and transport channels    -   Multiplexing/demultiplexing of MAC SDUs belonging to one or        different logical channels into/from transport blocks (TB)        delivered to/from the physical layer on transport channels    -   Scheduling information reporting    -   HARQ function (error correction through HARQ)    -   Priority handling between logical channels of one UE    -   Priority handling between UEs by means of dynamic scheduling    -   MBMS service identification    -   Transport format selection    -   Padding

The physical layers 220 and 225 channel-code and modulate upper layerdata, and convert the same into OFDM symbols that are then transmittedthrough a radio channel, or demodulate OFDM symbols received through aradio channel and channel-decode the same, and then transmit the same toupper layers.

FIG. 3 is a diagram illustrating the structure of a next-generationmobile communication system to which the disclosure may be applied.

Referring to FIG. 3 , a radio access network of a next-generation mobilecommunication system (hereinafter, referred to as “NR” or “5G”) mayinclude a new radio node B (hereinafter, referred to as an “NR gNB” oran “NR base station”) 310 and a new radio core network (hereinafter,referred to as an “NR CN”) 305. New radio user equipment (hereinafter,referred to as “NR UE” or a “terminal”) 315 accesses an external networkthrough the NR gNB 310 and the NR CN 305.

In FIG. 3 , the NR gNB 310 corresponds to an evolved Node B (eNB) in anexisting LTE system. The NR gNB 310 is connected to the NR UE 315through a radio channel 320, and may provide services superior to thoseof the existing node B. In the next-generation mobile communicationsystem, all user traffic is served through a shared channel. Therefore,a device for collecting status information, such as buffer status,available transmission power status, and channel status of UEs, andperforming scheduling is used. The NR NB 310 serves as such a device.

One NR gNB may control multiple cells. In order to realize super-highdata rates compared to the existing LTE system, the next-generationmobile communication system may have a bandwidth equal to or greaterthan the maximum bandwidth of the existing system. In addition, thenext-generation mobile communication system may use, as radio accesstechnology, orthogonal frequency division multiplexing (OFDM), and mayfurther employ a beamforming technique in addition thereto. In addition,an adaptive modulation and coding (hereinafter, referred to as “AMC”)scheme may be applied to determine a modulation scheme and a channelcoding rate in accordance with the channel status of a terminal.

The NR CN 305 performs functions such as mobility support, bearerconfiguration, and QoS configuration. The NR CN is a device thatperforms various control functions, as well as a mobility managementfunction for a terminal, and may be connected to a plurality of basestations. In addition, the next-generation mobile communication systemmay interwork with an existing LTE system, and the NR CN may beconnected to the MME 325 through a network interface. The MME may beconnected to the eNB 330, which is an existing base station.

FIG. 4 is a diagram illustrating a radio protocol structure of anext-generation mobile communication system to which the disclosure maybe applied.

Referring to FIG. 4 , the radio protocol of the next-generation mobilecommunication system includes NR SDAP 401 or 445, NR PDCP 405 or 440, NRRLC 410 or 435, and NR MAC 415 or 430 in a terminal and an NR basestation, respectively.

The primary functions of the NR SDAP 401 or 445 may include some of thefollowing functions.

-   -   Transfer of user plane data    -   Mapping between QoS flow and DRB for both downlink and uplink    -   Marking QoS flow ID in both downlink and uplink packets    -   Mapping reflective QoS flow to DRB for UL SDAP PDUs

With regard to the SDAP entity, the terminal may receive a configurationindicating whether or not to use a header of the SDAP entity or whetheror not to use functions of the SDAP entity for each PDCP entity, foreach bearer, or for each logical channel through an RRC message. In thecase where the SDAP header is configured, a 1-bit NAS reflective QoSconfiguration indicator and a 1-bit AS reflective QoS configurationindicator of the SDAP header may instruct the terminal to update orreconfigure mapping information between the QoS flow and the databearers in uplink and downlink. The SDAP header may include QoS flow IDinformation indicating the QoS. The QoS information may be used as dataprocessing priority, scheduling information, or the like in order tosupport effective services.

The primary functions of the NR PDCP 405 or 440 may include some of thefollowing functions.

-   -   Header compression and decompression (ROHC only)    -   Transfer of user data    -   In-sequence delivery of upper layer PDUs    -   Out-of-sequence delivery of upper layer PDUs    -   Sequence reordering (PDCP PDU reordering for reception)    -   Duplicate detection of lower layer SDUs    -   Retransmission of PDCP SDUs    -   Ciphering and deciphering    -   Timer-based SDU discard in uplink

The above reordering function of the NR PDCP entity denotes a functionof reordering PDCP PDUs received from a lower layer, based on a PDCPsequence number (SN), may include a function of transmitting data to anupper layer in the reordered order, may include a function of directlytransmitting data to an upper layer without consideration of an order,may include a function of reordering the sequence and recording lostPDCP PDUs, may include a function of sending a status report of the lostPDCP PDUs to the transmitting end, and may include a function of makinga request for retransmission of the lost PDCP PDUs.

The primary functions of the NR RLC 410 or 435 may include some of thefollowing functions.

-   -   Data transfer function (transfer of upper layer PDUs)    -   In-sequence delivery of upper layer PDUs    -   Out-of-sequence delivery of upper layer PDUs    -   ARQ function (error correction through ARQ)    -   Concatenation, segmentation, and reassembly of RLC SDUs    -   Re-segmentation of RLC data PDUs    -   Reordering of RLC data PDUs    -   Duplicate detection    -   Protocol error detection    -   RLC SDU discard    -   RLC re-establishment

The above in-sequence delivery function of the NR RLC entity denotes afunction of transferring RLC SDUs received from a lower layer to anupper layer in sequence, may include a function of, if one original RLCSDU is divided into a plurality of RLC SDUs and received, reassemblingand transmitting the same, may include a function of reordering thereceived RLC PDUs, based on an RLC sequence number (SN) or a PDCPsequence number (SN), may include a function of reordering the sequenceand recording lost RLC PDUs, may include a function of sending a statusreport of the lost RLC PDUs to the transmitting end, may include afunction of making a request for retransmission of the lost RLC PDUs,may include a function of, if there is a lost RLC SDU, transmitting onlythe RLC SDUs prior to the lost RLC SDU to an upper layer in sequence,may include a function of, if a predetermined timer expires even thoughthere is a lost RLC SDU, transmitting all RLC SDUs received before thetimer starts to an upper layer in sequence, or may include a functionof, if a predetermined timer expires even though there is a lost RLCSDU, transmitting all RLC SDUs received until the present to an upperlayer in sequence. In addition, the RLC PDUs may be processed in theorder of reception (in the order of arrival regardless of a serialnumber or a sequence number thereof), and may be transmitted to the PDCPentity in an out-of-sequence delivery manner. In the case of segments,the segments, which are stored in the buffer or will be received later,may be received and reconfigured into one complete RLC PDU, and the RLCPDU may be processed and transmitted to the PDCP entity. The NR RLClayer may not include a concatenation function, which may be performedin the NR MAC layer or may be replaced with a multiplexing function ofthe NR MAC layer.

The out-of-sequence delivery of the NR RLC entity denotes a function ofdirectly delivering RLC SDUs received from a lower layer to an upperlayer regardless of sequence, may include a function of, if one originalRLC SDU is divided into a plurality of RLC SDUs and is received,reassembling and delivering the same, and may include a function ofstoring and ordering RLC SNs or PDCP SNs of the received RLC PDUs,thereby recording the lost RLC PDUs.

The NR MAC 415 or 430 may be connected to a plurality of NR RLC entitiesconfigured in a single terminal, and the primary functions of the NR MACmay include some of the following functions.

-   -   Mapping between logical channels and transport channels    -   Multiplexing/demultiplexing of MAC SDUs    -   Scheduling information reporting    -   HARQ function (error correction through HARQ)    -   Priority handling between logical channels of one UE    -   Priority handling between UEs by means of dynamic scheduling    -   MBMS service identification    -   Transport format selection    -   Padding

The NR PHY layers 420 and 425 may perform operations of channel-codingand modulating the upper layer data into OFDM symbols and transmittingthe same through a radio channel, or operations of demodulating andchannel-decoding the OFDM symbols received through the radio channel andtransmitting the same to the upper layer.

FIG. 5 is a diagram illustrating the procedure in which a terminalswitches from an RRC idle mode to an RRC connected mode and establishesa connection with a network in the disclosure.

In FIG. 5 , if there is no transmission and reception of data to andfrom a terminal that transmits and receives data in an RRC connectedmode for a predetermined reason or for a performed period of time, abase station may transmit an “RRCConnectionRelease” message to theterminal such that the terminal switches to an RRC idle mode (501).

Afterwards, if a terminal that is not currently connected (hereinafter,referred to as an “idle mode UE”) has data to be transmitted, theterminal performs an RRC connection establishment process with the basestation. The terminal may establish reverse transmission synchronizationwith the base station through a random access process, and may transmitan “RRCConnectionRequest” message to the base station (505). The messagemay contain an identifier of the terminal, causes for establishing aconnection (establishmentCause), and the like. The base station maytransmit an “RRCConnectionSetup” message such that the terminalestablishes an RRC connection (510).

The message includes configuration information for eachservice/bearer/RLC device, for each logical channel, or for each bearer,information on whether or not to use ROHC for each bearer/logicalchannel, ROHC configuration information (e.g., information on the ROHCversion, initial information, etc.), “statusReportRequired” information(information through which the base station instructs the terminal toreport the PDCP status), “drb-ContinueROHC” information {this isconfiguration information instructing to maintain and use ROHCconfiguration information as that information is, and may be included inPDCP entity configuration information (pdcp-config) to then betransmitted}. In addition, the message may contain RRC connectionconfiguration information and the like. The bearer for RRC connection isalso called a “signaling radio bearer (SRB)”, and may be used intransmitting and receiving an RRC message, which is a control messagebetween the terminal and the base station.

The terminal that has established an RRC connection transmits an“RRCConnetionSetupComplete” message to the base station (515). Themessage may include a control message called “SERVICE REQUEST” throughwhich the terminal makes a request to the MME for configuring bearersfor predetermined services. The base station transmits message “SERVICEREQUEST” included in the “RRCConnetionSetupComplete” message to the MMEor AMF (520), and the MME or AMF determines whether or not to providethe service requested by the terminal.

As a result of the determination, if the terminal determines to providethe requested service, the MME or AMF may transmit an “INITIAL CONTEXTSETUP REQUEST” message to the base station (525). The message mayinclude information such as quality-of-service (QoS) information to beapplied when configuring a data radio bearer (DRB), and security-relatedinformation to be applied to the DRB (e.g., at least one of securitykeys, security algorithms, and the like may be included).

In addition, if the base station fails to receive capability informationof the terminal from the MME or AMF, the base station may transmit a UEcapability information request message to the terminal in order toidentify the capability information of the terminal (526). Uponreceiving the UE capability information request message, the terminalmay configure and produce a UE capability information message andtransmit the same to the base station (527).

The UE capability information message may include the type of handovermethod supported by the terminal. An indicator for each handover methodmay be defined, and the base station may identify the UE capabilityinformation, and may transmit, to the terminal, an indicator indicatingthe type of handover using a handover command message when instructinghandover.

The base station may exchange a “SecurityModeCommand” message (530) anda “SecurityModeComplete” message (535) with the terminal in order toestablish security. When the security is established, the base stationtransmits an “RRCConnectionReconfiguration” message to the terminal(540).

The message includes configuration information for eachservice/bearer/RLC device, for each logical channel, or for each bearer,information on whether or not to use ROHC for each bearer/logicalchannel, ROHC configuration information (e.g., information on the ROHCversion, initial information, etc.), “statusReportRequired” information(information through which the base station instructs the terminal toreport the PDCP status), “drb-ContinueROHC” information {this isconfiguration information instructing to maintain and use ROHCconfiguration information as that information is, and may be included inPDCP entity configuration information (pdcp-config) to then betransmitted}. In addition, the message contains RRC connectionconfiguration information and the like. The bearer for RRC connection isalso called a “signaling radio bearer (SRB)”, and is used intransmitting and receiving an RRC message, which is a control messagebetween the terminal and the base station.

In addition, the message includes configuration information on the DRBin which user data is processed, and the terminal configures a DRB byapplying the above information and transmits an“RRCConnectionReconfigurationComplete” message to the base station(545). After completing the DRB configuration with respect to theterminal, the base station transmits an “INITIAL CONTEXT SETUP COMPLETE”message to the MME or AMF (550). Upon receiving the message, the MME orAMF exchanges an “S1 BEARER SETUP” message and an “S1 BEARER SETUPRESPONSE” message with the S-GW in order to configure S1 bearer (555 and560). The S1 bearer is a connection for transmitting data, which isestablished between the S-GW and the base station, and corresponds tothe DRB in a one-to-one manner. When all of the above processes arecompleted, the terminal transmits and receives data to and from the basestation through the S-GW (565 and 570). The general data transmissionprocess described above fundamentally includes three steps: RRCconnection establishment, security configuration, and DRB configuration.In addition, the base station may transmit an RRC connectionreconfiguration message to the terminal in order to indicate a newconfiguration to the terminal, add another configuration, or modify theconfiguration for some reasons (575).

In the disclosure, the bearer may encompass an SRB and a DRB, the SRBmay denote “signaling radio bearer”, and the DRB may denote “data radiobearer”. The SRB is primarily used to transmit and receive RRC messagesof RRC entities, and the DRB is primarily used to transmit and receiveuser layer data. In addition, a UM DRB refers to the DRB using an RLCentity operating in an unacknowledged mode (UM), and an AM DRB refers tothe DRB using an RLC entity operating in an acknowledged mode (AM).

FIG. 6 is a diagram illustrating signaling procedures for performinghandover in a next-generation mobile communication system.

A terminal 601 in an RRC connected mode may transmit a cell measurementreport to the current source base station (source eNB) 602 periodicallyor when a specific event is satisfied (605).

The source base station may determine whether or not the terminal is toperform handover to an adjacent cell, based on the measurement report.“Handover” is a technique that switches the source base stationproviding services to the terminal in a connected mode to another basestation (or another cell of the same base station). If the source basestation determines handover, the source base station may requesthandover by transmitting a handover (HO) request message to a new basestation, that is, a “target base station (target eNB)” 603 that willprovide services to the terminal (610). If the target base stationaccepts the handover request, the target base station may transmit a “HOrequest Ack” message to the source base station (615).

Upon receiving the message, the source base station may transmit ahandover (HO) command message to the terminal (620). The source basestation may transmit the handover command message to the terminal usingan RRC connection reconfiguration message.

Upon receiving the message, the terminal may stop transmitting andreceiving data to and from the source base station, and may start atimer T304 (625). If the terminal fails to perform handover to thetarget base station for a predetermined period of time, the timer T304causes the terminal to return to its original configuration and switchto an RRC idle state. The source base station may transmit the sequencenumber (SN) status for uplink/downlink data to the target base station,and if there is downlink data, transmits the same to the target basestation (630 and 635).

The terminal may attempt random access to the target cell indicated bythe source base station (640). The random access is intended to informthe target cell that the terminal is moving through handover and tomatch the uplink synchronization. For the random access, the terminaltransmits, to the target cell, a preamble corresponding to the preambleID provided from the source base station or the preamble ID that israndomly selected.

After transmitting the preamble, the terminal may monitor whether or nota random access response (RAR) message is received from the target cellwhen a specific number of subframes elapses. The time period of themonitoring is called a “random access response window (RAR window)”.

If a random access response (RAR) is received during the specific periodof time (645), the terminal may transmit a handover (HO) completemessage to the target base station using an RRC reconfiguration completemessage (655). That is, the terminal may include information indicatingcompletion of handover in the RRC reconfiguration complete message, andmay transmit the same to the target base station.

Upon successfully receiving the random access response from the targetbase station as described above, the terminal may terminate the timerT304 (650). The target base station may request modification of pathsfrom an MME/S-GW/AMF 604 in order to modify paths of the bearers thatare configured for the source base station (660 and 665), and maytransmit a request for deleting UE context of the terminal to the sourcebase station (670). Accordingly, the terminal attempts to receive data(675) from the target base station from the start time of the RARwindow, and transmits an RRC reconfiguration complete message after theRAR is received, thereby starting transmission and reception of data toand from the target base station.

The disclosure proposes seamless handover methods capable of minimizingdata interruption time due to handover or reducing the same to 0 ms inthe next-generation mobile communication system.

The terminal is able to configure a plurality of first bearers withrespect to the source base station and perform transmission andreception of data (uplink or downlink data transmission/reception)through respective protocol entities of the bearers (PHY entities, MACentities, RLC entities, or PDCP entities), and hereinafter, an exampleof the terminal having one bearer will be illustrated and described inthe drawings and description for the convenience of description.

FIG. 7 is a diagram illustrating a first embodiment of an efficienthandover method for minimizing data interruption time due to handover inthe disclosure. Although described herein as various embodiments, suchas the first embodiment or the second embodiment, this descriptionshould not be construed as limiting. Various embodiments can be combinedor divided into separate embodiments without departing from the scope ofthe present disclosure.

Referring to FIG. 7 , in step 701, even if the terminal 720 receives ahandover command from the source base station 705 while transmitting andreceiving data to and from the source base station, the terminal maycontinue to transmit and receive data to and from the source basestation in order to minimize data interruption time that occurs duringhandover.

Therefore, in step 702, when the terminal 720 performs a random accessprocedure, transmits a preamble, or transmits initial data through anuplink transmission resource (e.g., a PUCCH or PUSCH transmissionresource) to the target base station 710 indicated by the handovercommand message, the terminal 720 may stop transmitting and receivingdata to and from the source base station (uplink data transmission anddownlink data reception).

In addition, in step 703, the terminal 720 may complete a procedure ofrandom access to the target base station, may transmit a handovercompletion message, and may start transmitting and receiving data to andfrom the target base station (uplink data transmission and downlink datareception).

FIG. 8 is a diagram illustrating second and third embodiments of anefficient handover method for minimizing data interruption time due tohandover in the disclosure.

Referring to FIG. 8 , in a second embodiment of the efficient handovermethod, in step 801, even if the terminal 820 receives a handovercommand from a source base station 805 while transmitting and receivingdata to and from the source base station, the terminal may continue totransmit and receive data to and from the source base station throughprotocol entities 822 of a first bearer in order to minimize datainterruption time that occurs during handover.

In addition, protocol entities (PHY entities, MAC entities, RLCentities, or PDCP entities) 821 of a second bearer for a target basestation may be configured or established in advance according to theconfiguration included in the received handover command message. Thesecond bearer may be configured and established so as to have the samebearer identifier as the first bearer in order to prevent the occurrenceof data interruption time for each bearer. In addition, even if theterminal receives a handover command message from the source basestation, the terminal may continue transmission and reception of datadue to HARQ retransmission in order to prevent loss of data, and thusmay not initialize a MAC entity of the first bearer.

In step 802, even when the terminal 820 performs a procedure of randomaccess to the target base station 810 indicated by the handover commandmessage through the protocol entities of the second bearer (for example,before a random access response is received after transmitting apreamble), the terminal may continue to transmit and receive data to andfrom the source base station (uplink data transmission and downlink datareception) through the protocol entities of the first bearer.

In addition, in step 803, the terminal 820 may complete a procedure ofrandom access to the target base station 810 through the protocolentities of the second bearer, and may transmit and receive data(downlink data reception and uplink data transmission).

In addition, if a first condition is satisfied, the terminal may stoptransmit and receiving data to and from the source base station 805through the protocol entities 822 of the first bearer. In addition, thePDCP entity 821 of the second bearer may continue to perform theseamless transmission and reception of data to and from the target basestation using information such as transmission/reception data, serialnumber information, header compression and decompression contexts, orthe like, which are stored in the PDCP entity 822 of the first bearer.The first condition may include at least one of the followingconditions.

-   -   When the terminal performs a procedure of random access to the        target base station through the entities 821 of the second        bearer and receives a random access response    -   when the terminal performs a procedure of random access to the        target base station through the entities of the second bearer,        receives a random access response, and configures and transmits        a handover completion message to the target base station    -   when the terminal completes a procedure of random access to the        target base station through the entities of the second bearer        and initially transmits data using a PUCCH or PUSCH uplink        transmission resource    -   when the base station configures a separate timer for the        terminal through an RRC message and the timer expires    -   the timer may start when the terminal receives a handover        command message from the source base station, when the terminal        starts random access to the target base station (when        transmitting a preamble), when the terminal receives a random        access response from the target base station, when the terminal        transmits a handover completion message to the target base        station, or when the terminal initially transmits data using a        PUCCH or PUSCH uplink transmission resource.    -   when the terminal performs a procedure of random access to the        target base station through the entities of the second bearer,        receives a random access response, and configures and transmits        a handover completion message to the target base station and        when the successful transmission of the handover completion        message is identified by a MAC entity (HARQ ACK) or an RLC        entity (RLC ACK)    -   when the terminal performs a procedure of random access to the        target base station through the entities of the second bearer,        receives a random access response, configures and transmits a        handover completion message to the target base station, and        receives an uplink transmission resource that is initially        allocated by the target base station, or receives an initial        indication of an uplink transmission resource    -   the source base station may determine the time to stop        transmitting the downlink data to the terminal or the time to        release the connection with the terminal, based on a        predetermined method when performing the efficient handover        proposed in the disclosure. For example, the predetermined        method may include at least one of when a predetermined timer        expires (the timer may start after indication of handover) or        when an indication indicating that the terminal has successfully        performed the handover to the target base station is received        from the target base station.

In addition, if no downlink data is received from the source basestation for a predetermined period of time, the terminal may determinethat the connection with the source base station is released, and mayrelease the connection.

-   -   When the terminal successfully completes a procedure of random        access to the target base station through the entities of the        second bearer, and receives a first uplink transmission resource        allocated by the target base station, or receives an indication        of a first uplink transmission resource    -   for example, more specifically, if the terminal is instructed to        perform random access to the target base station by receiving a        handover command message from the source base station, and if        the indicated random access is contention-free random access        (CFRA) {for example, if a predetermined preamble or a UE cell        identifier (e.g., C-RNTI) is allocated}        -   the terminal may determine that the random access procedure            is successfully completed when the terminal transmits a            predetermined preamble to the cell of the target base            station and receives a random access response (RAR) message.            Accordingly, the terminal may determine that the first            condition is satisfied when the terminal receives a first            uplink transmission resource allocated, included, or            indicated in the random access response message.    -   Alternatively, if the terminal is instructed to perform random        access to the target base station by receiving a handover        command message from the source base station, and if the        indicated random access is contention-based random access (CBRA)        {for example, if a predetermined preamble or a UE cell        identifier (e.g., C-RNTI) is not allocated)        -   the terminal may determine that the procedure of random            access to the target base station is successfully completed            when the terminal transmits a preamble (e.g., an arbitrary            preamble) to the cell of the target base station, receives a            random access response (RAR) message, transmits message 3            (e.g., a handover completion message) using the uplink            transmission resource allocated, included, or indicated in            the random access response message, and receives a MAC CE            (contention resolution MAC CE) indicating that the            contention has been resolved from the target base station.            Accordingly, the terminal may then determine that the first            condition is satisfied when the terminal monitors a PDCCH            and initially receives an uplink transmission resource            through the PDCCH corresponding to the C-RNTI of the            terminal or initially receives an indication thereof. As            another method, if the size of the uplink transmission            resource allocated by the random access response message is            sufficient enough for the terminal to further transmit            uplink data, as well as message 3, the terminal may            determine that an initial uplink transmission resource has            been received, thereby determining that the first condition            is satisfied.    -   In the case where a handover method that does not require a        random access procedure (RACH-less handover) is indicated by the        handover command message received by the terminal    -   if the handover command message includes an uplink transmission        resource to the target base station        -   the terminal may determine that the random access procedure            is successfully completed when the terminal transmits            message 3 (e.g., a handover completion message or an            “RRCReconfigurationComplete” message) using the uplink            transmission resource of the target base station and            receives a UE identity confirmation MAC CE from the base            station, thereby determining that the first condition is            satisfied. As another method, when the terminal receives a            first uplink transmission resource through a PDCCH            corresponding to a C-RNTI of the terminal by monitoring the            PDCCH after a random access procedure is successfully            completed, the terminal may determine that the first            condition is satisfied.    -   In the case where the handover command message does not include        an uplink transmission resource for the target base station        -   when the terminal receives an uplink transmission resource            through a PDCCH corresponding to a C-RNTI of the terminal by            monitoring the PDCCH for the target base station (or cell),            or when the terminal transmits message 3 (e.g., a handover            completion message or an “RRCReconfigurationComplete”            message) using the uplink transmission resource and receives            a UE identity confirmation MAC CE from the base station, the            terminal may determine that the random access procedure is            successfully completed, thereby determining that the first            condition is satisfied. As another method, when the terminal            receives a first uplink transmission resource through a            PDCCH corresponding to a C-RNTI of the terminal by            monitoring the PDCCH after a random access procedure is            successfully completed, the terminal may determine that the            first condition is satisfied.

Meanwhile, referring to FIG. 8 , in the third embodiment of theefficient handover method, in step 801, even if a terminal 820 receivesa handover command from the source base station 805 while transmittingand receiving data to and from the source base station, in order tominimize data interruption time that occurs during handover, theterminal may continue to transmit and receive data to and from thesource base station through protocol entities 822 of a first bearer.

In addition, protocol entities (PHY entities, MAC entities, RLCentities, or PDCP entities) 821 of the second bearer for the target basestation may be configured or established in advance according to theconfiguration included in the received handover command message. Thesecond bearer may be configured and established so as to have the samebearer identifier as the first bearer in order to prevent the occurrenceof data interruption time for each bearer.

In addition, the handover command message may include an uplinktransmission resource for the target base station, and may then transmitthe same to the terminal in order to omit the random access procedure ofthe terminal to the target base station through the protocol entities ofthe second bearer.

As another method, in the case where handover without a random accessprocedure is indicated by the handover command message, the terminal mayperform synchronization with the target base station without a randomaccess procedure, and may receive an uplink transmission resource bymonitoring a PDCCH from the target base station. As described above,data interruption time may be minimized by omitting the random accessprocedure.

In addition, even if the terminal receives a handover command messagefrom the source base station, the terminal may continue transmission andreception of data due to HARQ retransmission in order to prevent loss ofdata, and thus may not initialize a MAC entity of the first bearer. Inaddition, in the case of an RLC in an AM, the RLC retransmission may becontinuously performed.

In the third embodiment, in step 802, the operation in which theterminal 820 performs a procedure of random access to the target basestation 810 indicated by a handover command message through the protocolentities of the second bearer may be omitted. The terminal may configurea handover completion message, and may transmit the same to the targetbase station through the protocol entities of the second bearer using atransmission resource of the target base station indicated by thehandover command message.

As another method, in the case where the handover command messageindicates handover without a random access procedure but does notinclude an uplink transmission resource to the target base station, theterminal may perform synchronization with the target base stationwithout a random access procedure, and may receive an uplinktransmission resource by monitoring a PDCCH from the target basestation, thereby transmitting a handover completion message to thetarget base station. The terminal may continue transmitting andreceiving data to and from the source base station (uplink datatransmission and downlink data reception) through the protocol entitiesof the first bearer.

In the third embodiment, in step 803, the terminal 820 may transmit andreceive data (downlink data reception and uplink data transmission)using the protocol entities of the second bearer.

In addition, if a first condition is satisfied, the terminal may stoptransmitting and receiving data to and from the source base station 805through the protocol entities 822 of the first bearer. In addition, thePDCP entity 821 of the second bearer may continue to perform theseamless transmission and reception of data to and from the target basestation using information such as transmission/reception data, serialnumber information, header compression and decompression contexts, orthe like, which are stored in the PDCP entity 822 of the first bearer.The first condition may include at least one of the followingconditions.

-   -   When the terminal performs a procedure of random access to the        target base station through the entities of the second bearer,        receives a random access response, and configures and transmits        a handover completion message to the target base station    -   when the terminal completes a procedure of random access to the        target base station through the entities of the second bearer        and initially transmits data using a PUCCH or PUSCH uplink        transmission resource    -   when the base station configures a separate timer for the        terminal through an RRC message and the timer expires    -   the timer may start when the terminal receives a handover        command message from the source base station, when the terminal        starts random access to the target base station (when        transmitting a preamble), when the terminal receives a random        access response from the target base station, when the terminal        transmits a handover completion message to the target base        station, or when the terminal initially transmits data using a        PUCCH or PUSCH uplink transmission resource.    -   when the terminal performs a procedure of random access to the        target base station through the entities of the second bearer,        receives a random access response, and configures and transmits        a handover completion message to the target base station and        when the successful transmission of the handover completion        message is identified by a MAC entity (HARQ ACK) or an RLC        entity (RLC ACK)    -   when the terminal performs a procedure of random access to the        target base station through the entities of the second bearer,        receives a random access response, configures and transmits a        handover completion message to the target base station, and        receives an uplink transmission resource that is initially        allocated by the target base station, or receives an initial        indication of an uplink transmission resource    -   the source base station may determine the time to stop        transmitting the downlink data to the terminal or the time to        release the connection with the terminal, based on a        predetermined method when performing the efficient handover        proposed in the disclosure. For example, the predetermined        method may include at least one of when a predetermined timer        expires (the timer may start after indication of handover) or        when an indication indicating that the terminal has successfully        performed the handover to the target base station is received        from the target base station.

In addition, if no downlink data is received from the source basestation for a predetermined period of time, the terminal may determinethat the connection with the source base station is released, and mayrelease the connection.

-   -   When the terminal successfully completes a procedure of random        access to the target base station through the entities of the        second bearer, and receives a first uplink transmission resource        allocated by the target base station, or receives an indication        of a first uplink transmission resource    -   for example, more specifically, if the terminal is instructed to        perform random access to the target base station by receiving a        handover command message from the source base station, and if        the indicated random access is contention-free random access        (CFRA) {for example, if a predetermined preamble or a UE cell        identifier (e.g., C-RNTI) is allocated}        -   when the terminal transmits a predetermined preamble to the            cell of the target base station and receives a random access            response (RAR) message, the terminal may determine that the            random access procedure is successfully completed.            Accordingly, when the terminal receives a first uplink            transmission resource allocated, included, or indicated in            the random access response message, the terminal may            determine that the first condition is satisfied.    -   Alternatively, if the terminal is instructed to perform random        access to the target base station by receiving a handover        command message from the source base station, and if the        indicated random access is contention-based random access (CBRA)        {for example, if a predetermined preamble or a UE cell        identifier (e.g., C-RNTI) is not allocated)        -   the terminal may determine that the procedure of random            access to the target base station is successfully completed            when the terminal transmits a preamble (e.g., an arbitrary            preamble) to the cell of the target base station, receives a            random access response (RAR) message, transmits message 3            (e.g., a handover completion message) using the uplink            transmission resource allocated, included, or indicated in            the random access response message, and receives a MAC CE            (contention resolution MAC CE) indicating that the            contention has been resolved from the target base station.            Accordingly, when, thereafter, the terminal monitors the            PDCCH and initially receives an uplink transmission resource            through the PDCCH corresponding to the C-RNTI of the            terminal or initially receives an indication thereof, the            terminal may determine that the first condition is            satisfied. As another method, if the size of the uplink            transmission resource allocated by the random access            response message is sufficient enough for the terminal to            further transmit uplink data, as well as message 3, the            terminal may determine that an initial uplink transmission            resource has been received, thereby determining that the            first condition is satisfied.    -   In the case where a handover method that does not require a        random access procedure (RACH-less handover) is indicated by the        handover command message received by the terminal    -   if the handover command message includes an uplink transmission        resource to the target base station        -   the terminal may determine that the random access procedure            is successfully completed when the terminal transmits            message 3 (e.g., a handover completion message or an            “RRCReconfigurationComplete” message) using the uplink            transmission resource of the target base station and            receives a UE identity confirmation MAC CE from the base            station, thereby determining that the first condition is            satisfied. As another method, when the terminal receives a            first uplink transmission resource through a PDCCH            corresponding to a C-RNTI of the terminal by monitoring the            PDCCH after a random access procedure is successfully            completed, the terminal may determine that the first            condition is satisfied.    -   In the case where the handover command message does not include        an uplink transmission resource for the target base station        -   when the terminal receives an uplink transmission resource            through a PDCCH corresponding to a C-RNTI of the terminal by            monitoring the PDCCH for the target base station (or cell),            or when the terminal transmits message 3 (e.g., a handover            completion message or an “RRCReconfigurationComplete”            message) using the uplink transmission resource and receives            a UE identity confirmation MAC CE from the base station, the            terminal may determine that the random access procedure is            successfully completed, thereby determining that the first            condition is satisfied. As another method, when the terminal            receives a first uplink transmission resource through a            PDCCH corresponding to a C-RNTI of the terminal by            monitoring the PDCCH after a random access procedure is            successfully completed, the terminal may determine that the            first condition is satisfied.

FIG. 9 is a diagram illustrating a fourth embodiment of an efficienthandover method for minimizing data interruption time due to handover inthe disclosure.

Referring to FIG. 9 , in step 901, even if the terminal 920 receives ahandover command from a source base station 905 while transmitting andreceiving data to and from the source base station, the terminal maycontinue to transmit and receive data to and from the source basestation through protocol entities 922 of a first bearer in order tominimize data interruption time that occurs during handover.

In addition, protocol entities (PHY entities, MAC entities, RLCentities, or PDCP entities) 921 of a second bearer for the target basestation may be configured or established in advance according to theconfiguration included in the received handover command message. Thesecond bearer may be configured and established so as to have the samebearer identifier as the first bearer in order to prevent the occurrenceof data interruption time for each bearer.

In addition, in the fourth embodiment, the PDCP entity of the firstbearer and the PDCP entity of the second bearer may be logicallyoperated as a single PDCP entity, and a more detailed operation methodthereof will be described with reference to FIG. 11 .

In addition, in the case where the terminal is allowed to transmituplink data both to the source base station and to the target basestation, there may be a problem with reduction in coverage due toinsufficient transmission power of the terminal or with determination ofthe base station to which a request for a transmission resource is to bemade and uplink data is to be transmitted when transmitting uplink data(link selection problem). Therefore, in order to avoid the aboveproblem, the terminal may transmit uplink data to the source basestation and the target base station at different times through a timedomain multiplexing (TDM) scheme when transmitting uplink data to thesource base station or the target base station in the fourth embodiment.

As another method, the base station may configure a threshold through anRRC message, and the terminal may transmit uplink data to the sourcebase station (or the target base station) if the size of the uplink dataof the terminal is smaller than the threshold, and may transmit uplinkdata both to the source base station and to target base station if thesize of the uplink data of the terminal is greater than the threshold.The transmission of uplink data may accompany a buffer status report(BSR) procedure for reporting the size of data to be transmitted or ascheduling request procedure.

In addition, even if the terminal receives a handover command messagefrom the source base station, the terminal may continue transmission andreception of data due to HARQ retransmission in order to prevent loss ofdata, and thus may not initialize a MAC entity of the first bearer. Inaddition, in the case of an RLC entity in an AM, the RLC retransmissionmay be continuously performed.

In step 902, the terminal 920 may continue to transmit and receive datato and from the source base station (uplink data transmission anddownlink data transmission) through the protocol entities of the firstbearer of the terminal even when performing a procedure of random accessto the target base station 910 indicated by the handover command messagethrough the protocol entities of the second bearer.

In step 903, the terminal 920 may complete a procedure of random accessto the target base station 910 through the protocol entities of thesecond bearer, may transmit and receive data (uplink data transmissionand downlink data transmission), and may continue to transmit andreceive data to and from the source base station (uplink datatransmission and downlink data transmission) through the protocolentities of the first bearer.

In step 904, the terminal 920 may stop transmitting and receiving datato and from the source base station 805 through the protocol entities922 of the first bearer if a first condition is satisfied. In addition,the PDCP entity 921 of the second bearer may continue to perform theseamless transmission and reception of data to and from the target basestation using information such as transmission/reception data, serialnumber information, header compression and decompression contexts, orthe like, which are stored in the PDCP entity 922 of the first bearer.The first condition may include at least one of the followingconditions.

-   -   When the terminal performs a procedure of random access to the        target base station through the entities 921 of the second        bearer and receives a random access response    -   when the terminal performs a procedure of random access to the        target base station through the entities of the second bearer,        receives a random access response, and configures and transmits        a handover completion message to the target base station    -   when the terminal completes a procedure of random access to the        target base station through the entities of the second bearer        and initially transmits data using a PUCCH or PUSCH uplink        transmission resource    -   when the base station configures a separate timer for the        terminal through an RRC message and the timer expires    -   the timer may start when the terminal receives a handover        command message from the source base station, when the terminal        starts random access to the target base station (when        transmitting a preamble), when the terminal receives a random        access response from the target base station, when the terminal        transmits a handover completion message to the target base        station, or when the terminal initially transmits data using a        PUCCH or PUSCH uplink transmission resource.    -   when the terminal performs a procedure of random access to the        target base station through the entities of the second bearer,        receives a random access response, and configures and transmits        a handover completion message to the target base station and        when the successful transmission of the handover completion        message is identified by a MAC entity (HARQ ACK) or an RLC        entity (RLC ACK)    -   when the terminal performs a procedure of random access to the        target base station through the entities of the second bearer,        receives a random access response, configures and transmits a        handover completion message to the target base station, and        receives an uplink transmission resource that is initially        allocated by the target base station, or receives an initial        indication of an uplink transmission resource    -   the source base station may determine the time to stop        transmitting the downlink data to the terminal or the time to        release the connection with the terminal, based on a        predetermined method when performing the efficient handover        proposed in the disclosure. For example, the predetermined        method may include at least one of when a predetermined timer        expires (the timer may start after indication of handover) or        when an indication indicating that the terminal has successfully        performed the handover to the target base station is received        from the target base station.

In addition, if no downlink data is received from the source basestation for a predetermined period of time, the terminal may determinethat the connection with the source base station is released, and mayrelease the connection.

-   -   When the terminal receives, from the target base station, an        indication {e.g., an RRC message (e.g., an “RRCReconfiguration”        message), a MAC CE, an RLC control PDU, or a PDCP control PDU}        to release the connection with the source base station    -   when the terminal receives, from the source base station, an        indication {e.g., an RRC message (e.g., an “RRCReconfiguration”        message), a MAC CE, an RLC control PDU, or a PDCP control PDU}        to release the connection with the source base station    -   when the terminal successfully completes a procedure of random        access to the target base station through the entities of the        second bearer, and receives a first uplink transmission resource        allocated by the target base station, or receives an indication        of a first uplink transmission resource    -   for example, more specifically, if the terminal is instructed to        perform random access to the target base station by receiving a        handover command message from the source base station, and if        the indicated random access is contention-free random access        (CFRA) {for example, if a predetermined preamble or a UE cell        identifier (e.g., C-RNTI) is allocated}        -   the terminal may determine that the random access procedure            is successfully completed when the terminal transmits a            predetermined preamble to the cell of the target base            station and receives a random access response (RAR) message.            Accordingly, when the terminal receives a first uplink            transmission resource allocated, included, or indicated in            the random access response message, the terminal may            determine that the first condition is satisfied.    -   If the terminal is instructed to perform random access to the        target base station by receiving a handover command message from        the source base station, and if the indicated random access is        contention-based random access (CBRA) {for example, if a        predetermined preamble or a UE cell identifier (e.g., C-RNTI) is        not allocated)        -   the terminal may determine that the procedure of random            access to the target base station is successfully completed            when the terminal transmits a preamble (e.g., an arbitrary            preamble) to the cell of the target base station, receives a            random access response (RAR) message, transmits message 3            (e.g., a handover completion message) using the uplink            transmission resource allocated, included, or indicated in            the random access response message, and receives a MAC CE            (contention resolution MAC CE) indicating that the            contention has been resolved from the target base station.            Accordingly, thereafter, when the terminal monitors a PDCCH            and initially receives an uplink transmission resource            through the PDCCH corresponding to the C-RNTI of the            terminal or initially receives an indication thereof, the            terminal may determine that the first condition is            satisfied. As another method, if the size of the uplink            transmission resource allocated by the random access            response message is sufficient enough for the terminal to            further transmit uplink data, as well as message 3, the            terminal may determine that an initial uplink transmission            resource has been received, thereby determining that the            first condition is satisfied.    -   In the case where a handover method that does not require a        random access procedure (RACH-less handover) is indicated by the        handover command message received by the terminal    -   if the handover command message includes an uplink transmission        resource to the target base station        -   the terminal may determine that the random access procedure            is successfully completed when the terminal transmits            message 3 (e.g., a handover completion message or an            “RRCReconfigurationComplete” message) using the uplink            transmission resource of the target base station and            receives a UE identity confirmation MAC CE from the base            station, thereby determining that the first condition is            satisfied. As another method, when the terminal receives a            first uplink transmission resource through a PDCCH            corresponding to a C-RNTI of the terminal by monitoring the            PDCCH after a random access procedure is successfully            completed, the terminal may determine that the first            condition is satisfied.    -   In the case where the handover command message does not include        an uplink transmission resource for the target base station        -   when the terminal receives an uplink transmission resource            through a PDCCH corresponding to a C-RNTI of the terminal by            monitoring the PDCCH for the target base station (or cell),            or when the terminal transmits message 3 (e.g., a handover            completion message or an “RRCReconfigurationComplete”            message) using the uplink transmission resource and receives            a UE identity confirmation MAC CE from the base station, the            terminal may determine that the random access procedure is            successfully completed, thereby determining that the first            condition is satisfied. As another method, when the terminal            receives a first uplink transmission resource through a            PDCCH corresponding to a C-RNTI of the terminal by            monitoring the PDCCH after a random access procedure is            successfully completed, the terminal may determine that the            first condition is satisfied.

FIG. 10 is a diagram illustrating a fifth embodiment of an efficienthandover method for minimizing data interruption time due to handover inthe disclosure.

Referring to FIG. 10 , in a fifth embodiment, in step 1001, even if theterminal 1020 receives a handover command from a source base station1005 while transmitting and receiving data to and from the source basestation, the terminal may continue to transmit and receive data to andfrom the source base station through protocol entities 1022 of a firstbearer in order to minimize data interruption time that occurs duringhandover.

In addition, protocol entities (PHY entities, MAC entities, RLCentities, or PDCP entities) 1021 of a second bearer for the target basestation may be configured or established in advance according to theconfiguration included in the received handover command message. Thesecond bearer may be configured and established so as to have the samebearer identifier as the first bearer in order to prevent the occurrenceof data interruption time for each bearer.

In addition, in the fifth embodiment, the PDCP entity of the firstbearer and the PDCP entity of the second bearer may be logicallyoperated as a single PDCP entity, and a more detailed operation methodthereof will be described with reference to FIG. 11 .

In addition, in the case where the terminal is allowed to transmituplink data both to the source base station and to the target basestation in the fifth embodiment, there may be a problem with reductionin coverage due to insufficient transmission power of the terminal orwith determination of the base station to which a request for atransmission resource is to be made and uplink data is to be transmittedwhen transmitting uplink data (link selection problem). Therefore, inorder to avoid the above problem, the terminal may transmit uplink datato only one of the source base station or the target base station in thefifth embodiment.

Therefore, the terminal may make a request for scheduling to only one ofthe source base station and the target base station, may report the sizeof data to be transmitted in the PDCP entity {e.g., a buffer statusreport (BSR)} to only one of the source base station and the target basestation, may receive an uplink transmission resource, and may thentransmit uplink data to only one base station. In addition, even if theterminal receives a handover command message from the source basestation, the terminal may continue transmission and reception of datadue to HARQ retransmission in order to prevent loss of data, and thusmay not initialize a MAC entity of the first bearer. In addition, in thecase of an RLC entity in an AM, the RLC retransmission may becontinuously performed.

In the fifth embodiment, in step 1002, the terminal 1020 may continue totransmit and receive data to and from the source base station (uplinkdata transmission and downlink data transmission) through the protocolentities of the first bearer even when performing a procedure of randomaccess to the target base station 1010 indicated by the handover commandmessage through the protocol entities of the second bearer.

In the fifth embodiment, in step 1003, the terminal 1020 may stoptransmitting and receiving data to and from the source base stationthrough the protocol entities 1022 of the first bearer if a secondcondition is satisfied, may transmit uplink data to the target basestation through the protocol entities 1021 of the second bearer, and maycontinue receiving downlink data from the source base station and thetarget base station through the protocol entities of the first bearerand the second bearer. In addition, the PDCP entity 1021 of the secondbearer may continue to perform the seamless transmission and receptionof data to and from the target base station using information such astransmission/reception data, serial number information, headercompression and decompression contexts, or the like, which are stored inthe PDCP entity 1022 of the first bearer.

The second condition may include at least one of the followingconditions.

-   -   When the terminal performs a procedure of random access to the        target base station through the entities 1021 of the second        bearer and receives a random access response    -   when the terminal performs a procedure of random access to the        target base station through the entities of the second bearer,        receives a random access response, and configures and transmits        a handover completion message to the target base station    -   when the terminal completes a procedure of random access to the        target base station through the entities of the second bearer        and initially transmits data using a PUCCH or PUSCH uplink        transmission resource    -   when the base station configures a separate timer for the        terminal through an RRC message and the timer expires    -   the timer may start when the terminal receives a handover        command message from the source base station, when the terminal        starts random access to the target base station (when        transmitting a preamble), when the terminal receives a random        access response from the target base station, when the terminal        transmits a handover completion message to the target base        station, or when the terminal initially transmits data using a        PUCCH or PUSCH uplink transmission resource.    -   when the terminal performs a procedure of random access to the        target base station through the entities of the second bearer,        receives a random access response, and configures and transmits        a handover completion message to the target base station and        when the successful transmission of the handover completion        message is identified by a MAC entity (HARQ ACK) or an RLC        entity (RLC ACK)    -   when the terminal performs a procedure of random access to the        target base station through the entities of the second bearer,        receives a random access response, configures and transmits a        handover completion message to the target base station, and        receives an uplink transmission resource that is initially        allocated by the target base station, or receives an initial        indication of an uplink transmission resource    -   when the terminal receives, from the target base station, an        indication {e.g., an RRC message (e.g., an “RRCReconfiguration”        message), a MAC CE, an RLC control PDU, or a PDCP control PDU}        to release the connection with the source base station and to        switch the uplink to the target base station    -   when the terminal receives, from the source base station, an        indication {e.g., an RRC message (e.g., an “RRCReconfiguration”        message), a MAC CE, an RLC control PDU, or a PDCP control PDU}        to release the connection with the source base station and to        switch the uplink to the target base station    -   when the terminal successfully completes a procedure of random        access to the target base station through the entities of the        second bearer, and receives a first uplink transmission resource        allocated by the target base station, or receives an indication        of a first uplink transmission resource    -   for example, more specifically, if the terminal is instructed to        perform random access to the target base station by receiving a        handover command message from the source base station, and if        the indicated random access is contention-free random access        (CFRA) {for example, if a predetermined preamble or a UE cell        identifier (e.g., C-RNTI) is allocated}        -   the terminal may determine that the random access procedure            is successfully completed when the terminal transmits a            predetermined preamble to the cell of the target base            station and receives a random access response (RAR) message.            Accordingly, when the terminal receives a first uplink            transmission resource allocated, included, or indicated in            the random access response message, the terminal may            determine that the second condition is satisfied.    -   If the terminal is instructed to perform random access to the        target base station by receiving a handover command message from        the source base station, and if the indicated random access is        contention-based random access (CBRA) {for example, if a        predetermined preamble or a UE cell identifier (e.g., C-RNTI) is        not allocated)        -   the terminal may determine that the procedure of random            access to the target base station is successfully completed            when the terminal transmits a preamble (e.g., an arbitrary            preamble) to the cell of the target base station, receives a            random access response (RAR) message, transmits message 3            (e.g., a handover completion message) using the uplink            transmission resource allocated, included, or indicated in            the random access response message, and receives a MAC CE            (contention resolution MAC CE) indicating that the            contention has been resolved from the target base station.            Accordingly, thereafter, when the terminal monitors a PDCCH            and initially receives an uplink transmission resource            through the PDCCH corresponding to the C-RNTI of the            terminal or initially receives an indication thereof, the            terminal may determine that the second condition is            satisfied. As another method, if the size of the uplink            transmission resource allocated by the random access            response message is sufficient enough for the terminal to            further transmit uplink data, as well as message 3, the            terminal may determine that an initial uplink transmission            resource has been received, thereby determining that the            second condition is satisfied.    -   In the case where a handover method that does not require a        random access procedure (RACH-less handover) is indicated by the        handover command message received by the terminal    -   if the handover command message includes an uplink transmission        resource to the target base station        -   the terminal may determine that the random access procedure            is successfully completed when the terminal transmits            message 3 (e.g., a handover completion message or an            “RRCReconfigurationComplete” message) using the uplink            transmission resource of the target base station and            receives a UE identity confirmation MAC CE from the base            station, thereby determining that the second condition is            satisfied. As another method, when the terminal receives a            first uplink transmission resource through a PDCCH            corresponding to a C-RNTI of the terminal by monitoring the            PDCCH after a random access procedure is successfully            completed, the terminal may determine that the second            condition is satisfied.    -   In the case where the handover command message does not include        an uplink transmission resource for the target base station        -   when the terminal receives an uplink transmission resource            through a PDCCH corresponding to a C-RNTI of the terminal by            monitoring the PDCCH for the target base station (or cell),            or when the terminal transmits message 3 (e.g., a handover            completion message or an “RRCReconfigurationComplete”            message) using the uplink transmission resource and receives            a UE identity confirmation MAC CE from the base station, the            terminal may determine that the random access procedure is            successfully completed, thereby determining that the second            condition is satisfied. As another method, when the terminal            receives a first uplink transmission resource through a            PDCCH corresponding to a C-RNTI of the terminal by            monitoring the PDCCH after a random access procedure is            successfully completed, the terminal may determine that the            second condition is satisfied.

In the case where the terminal is able to continue to receive downlinkdata from the source base station and the target base station throughthe protocol entities of the first bearer and the second bearer, inorder to facilitate reception of downlink data from the source basestation (or the target base station) or facilitate the source basestation (or the target base station) to transmit downlink data, for AMbearers, an RLC status report other than data is allowed to becontinuously transmitted to the source base station (or the target basestation) in uplink through the protocol entities of the first bearer (orthe second bearer). This is due to the fact that if an RLC status reportof successful delivery is not indicated (that is, if the RLC statusreport is not received) after the AM bearers transmit data to atransmitting end, the AM bearers are unable to continue to transmit datathereafter.

In addition, in the fifth embodiment, in step 1003, even if the terminal1020 stops transmitting uplink data to the source base station throughthe protocol entities 1022 of the first bearer because the secondcondition is satisfied, and makes a switch to start transmitting uplinkdata to the target base station through the protocol entities 1021 ofthe second bearer, in order to facilitate reception of downlink datafrom the source base station (or the target base station) or facilitatethe source base station (or the target base station) to transmitdownlink data, the terminal may allow continuous transmission of HARQACK or HARQ NACK information or PDCP control data (for example, a PDCPstatus report or ROHC feedback information) through the protocolentities of the first bearer (or the second bearer).

In addition, in the fifth embodiment, in step 1003, even if the terminal1020 stops transmitting uplink data to the source base station throughthe protocol entities 1022 of the first bearer because the secondcondition is satisfied, and makes a switch to start transmitting uplinkdata to the target base station through the protocol entities 1021 ofthe second bearer, the terminal may continue transmission of data due toretransmission of HARQ of a MAC entity or transmission of data due toretransmission of an RLC entity in an AM in order to prevent loss ofdata to the source base station.

In addition, in the fifth embodiment, in step 1003, if the terminal 1020stops transmitting uplink data to the source base station through theprotocol entities 1022 of the first bearer because the second conditionis satisfied, and makes a switch to start transmitting uplink data tothe target base station through the protocol entities 1021 of the secondbearer, the source base station or the target base station may allocatetransmission resources to the terminal by dividing time such that theuplink transmission resource to the target base station and the uplinktransmission resource to the source base station do not collide with (ordo not overlap) each other. If the uplink transmission resource to thetarget base station overlaps the uplink transmission resource to thesource base station, the terminal may prioritize the uplink transmissionresource to the source base station in order to maintain transmission ofdownlink data from the source base station, thereby performing thetransmission of data to the source base station.

Alternatively, if the uplink transmission resource to the target basestation overlaps the uplink transmission resource to the source basestation, the terminal may prioritize the uplink transmission resource tothe target base station in order to maintain transmission of downlinkdata from the target base station, thereby performing transmission ofdata to the target base station.

Specifically, in the case where handover corresponding to the fifthembodiment of the disclosure is indicated when receiving a handovercommand message, the terminal may perform scheduling request through afirst protocol entity until the second condition is satisfied, maytransmit a buffer status report to the source base station, may receivean uplink transmission resource, may transmit uplink data, and mayreceive downlink data from the source base station.

However, if the second condition is satisfied, the terminal may nolonger transmit data to the source base station, may switch the uplinkto perform scheduling request through the second protocol entity, maytransmit a buffer status report to the target base station, may receivean uplink transmission resource, and may transmit uplink data to thetarget base station.

However, the terminal may continue to receive downlink data from thesource base station, and may continue to transmit HARQ ACK, HARQ NACK,an RLC status report, or PDCP control data (for example, a PDCP statusreport or ROHC feedback information) corresponding to the downlink data.The terminal may also receive downlink data from the target base stationif the second condition is satisfied.

In the fifth embodiment, in step 1004, the terminal 1020 may stopreceiving downlink data from the source base station 1005 through theprotocol entities 1022 of the first bearer if a first condition issatisfied. In addition, the PDCP entity 1021 of the second bearer maycontinue to perform the seamless transmission and reception of data toand from the target base station using information such astransmission/reception data, serial number information, headercompression and decompression contexts, or the like, which are stored inthe PDCP entity 1022 of the first bearer. The first condition mayinclude at least one of the following conditions.

-   -   When the terminal performs a procedure of random access to the        target base station through the entities 1021 of the second        bearer and receives a random access response    -   when the terminal performs a procedure of random access to the        target base station through the entities of the second bearer,        receives a random access response, and configures and transmits        a handover completion message to the target base station    -   when the terminal completes a procedure of random access to the        target base station through the entities of the second bearer        and initially transmits data using a PUCCH or PUSCH uplink        transmission resource    -   when the base station configures a separate timer for the        terminal through an RRC message and the timer expires    -   the timer may start when the terminal receives a handover        command message from the source base station, when the terminal        starts random access to the target base station (when        transmitting a preamble), when the terminal receives a random        access response from the target base station, when the terminal        transmits a handover completion message to the target base        station, or when the terminal initially transmits data using a        PUCCH or PUSCH uplink transmission resource.    -   when the terminal performs a procedure of random access to the        target base station through the entities of the second bearer,        receives a random access response, and configures and transmits        a handover completion message to the target base station and        when the successful transmission of the handover completion        message is identified by a MAC entity (HARQ ACK) or an RLC        entity (RLC ACK)    -   when the terminal performs a procedure of random access to the        target base station through the entities of the second bearer,        receives a random access response, configures and transmits a        handover completion message to the target base station, and        receives an uplink transmission resource that is initially        allocated by the target base station, or receives an initial        indication of an uplink transmission resource    -   the source base station may determine the time to stop        transmitting the downlink data to the terminal or the time to        release the connection with the terminal, based on a        predetermined method when performing the efficient handover        proposed in the disclosure. For example, the predetermined        method may include at least one of when a predetermined timer        expires (the timer may start after indication of handover) or        when an indication indicating that the terminal has successfully        performed the handover to the target base station is received        from the target base station.

In addition, if no downlink data is received from the source basestation for a predetermined period of time, the terminal may determinethat the connection with the source base station is released, and mayrelease the connection.

-   -   When the terminal receives, from the target base station, an        indication {e.g., an RRC message (e.g., an “RRCReconfiguration”        message), a MAC CE, an RLC control PDU, or a PDCP control PDU}        to release the connection with the source base station    -   when the terminal receives, from the source base station, an        indication {e.g., an RRC message (e.g., an “RRCReconfiguration”        message), a MAC CE, an RLC control PDU, or a PDCP control PDU}        to release the connection with the source base station    -   when the terminal fails to receive downlink data from the source        base station for a predetermined period of time    -   when the terminal successfully completes a procedure of random        access to the target base station through the entities of the        second bearer, and receives a first uplink transmission resource        allocated by the target base station, or receives an indication        of a first uplink transmission resource    -   for example, more specifically, if the terminal is instructed to        perform random access to the target base station by receiving a        handover command message from the source base station, and if        the indicated random access is contention-free random access        (CFRA) {for example, if a predetermined preamble or a UE cell        identifier (e.g., C-RNTI) is allocated}        -   the terminal may determine that the random access procedure            is successfully completed when the terminal transmits a            predetermined preamble to the cell of the target base            station and receives a random access response (RAR) message.            Accordingly, when the terminal receives a first uplink            transmission resource allocated, included, or indicated in            the random access response message, the terminal may            determine that the first condition is satisfied.    -   If the terminal is instructed to perform random access to the        target base station by receiving a handover command message from        the source base station, and if the indicated random access is        contention-based random access (CBRA) {for example, if a        predetermined preamble or a UE cell identifier (e.g., C-RNTI) is        not allocated)        -   the terminal may determine that the procedure of random            access to the target base station is successfully completed            when the terminal transmits a preamble (e.g., an arbitrary            preamble) to the cell of the target base station, receives a            random access response (RAR) message, transmits message 3            (e.g., a handover completion message) using the uplink            transmission resource allocated, included, or indicated in            the random access response message, and receives a MAC CE            (contention resolution MAC CE) indicating that the            contention has been resolved from the target base station.            Accordingly, thereafter, when the terminal monitors a PDCCH            and initially receives an uplink transmission resource            through the PDCCH corresponding to the C-RNTI of the            terminal or initially receives an indication thereof, the            terminal may determine that the first condition is            satisfied. As another method, if the size of the uplink            transmission resource allocated by the random access            response message is sufficient enough for the terminal to            further transmit uplink data, as well as message 3, the            terminal may determine that an initial uplink transmission            resource has been received, thereby determining that the            first condition is satisfied.    -   In the case where a handover method that does not require a        random access procedure (RACH-less handover) is indicated by the        handover command message received by the terminal    -   if the handover command message includes an uplink transmission        resource to the target base station        -   the terminal may determine that the random access procedure            is successfully completed when the terminal transmits            message 3 (e.g., a handover completion message or an            “RRCReconfigurationComplete” message) using the uplink            transmission resource of the target base station and            receives a UE identity confirmation MAC CE from the base            station, thereby determining that the first condition is            satisfied. As another method, when the terminal receives a            first uplink transmission resource through a PDCCH            corresponding to a C-RNTI of the terminal by monitoring the            PDCCH after a random access procedure is successfully            completed, the terminal may determine that the first            condition is satisfied.    -   In the case where the handover command message does not include        an uplink transmission resource for the target base station        -   when the terminal receives an uplink transmission resource            through a PDCCH corresponding to a C-RNTI of the terminal by            monitoring the PDCCH for the target base station (or cell),            or when the terminal transmits message 3 (e.g., a handover            completion message or an “RRCReconfigurationComplete”            message) using the uplink transmission resource and receives            a UE identity confirmation MAC CE from the base station, the            terminal may determine that the random access procedure is            successfully completed, thereby determining that the first            condition is satisfied. As another method, when the terminal            receives a first uplink transmission resource through a            PDCCH corresponding to a C-RNTI of the terminal by            monitoring the PDCCH after a random access procedure is            successfully completed, the terminal may determine that the            first condition is satisfied.

FIG. 10 may illustrate specific steps in a 5-2^(nd) embodiment of anefficient handover method for minimizing data interruption time due tohandover in the disclosure.

Referring to FIG. 10 , in a 5-2^(nd) embodiment, in step 1001, even ifthe terminal 1020 receives a handover command from a source base station1005 while transmitting and receiving data to and from the source basestation, the terminal may continue to transmit and receive data to andfrom the source base station through protocol entities 1022 of a firstbearer in order to minimize data interruption time that occurs duringhandover.

In addition, protocol entities (PHY entities, MAC entities, RLCentities, or PDCP entities) 1021 of a second bearer for the target basestation may be configured or established in advance according to theconfiguration included in the received handover command message. Thesecond bearer may be configured and established so as to have the samebearer identifier as the first bearer in order to prevent the occurrenceof data interruption time for each bearer.

In addition, in the 5-2^(nd) embodiment, the PDCP entity of the firstbearer and the PDCP entity of the second bearer may be logicallyoperated as a single PDCP entity, and a more detailed operation methodthereof will be described with reference to FIG. 11 .

In addition, in the 5-2^(nd) embodiment, In addition, in the case wherethe terminal is allowed to transmit uplink data both to the source basestation and to the target base station, there may be a problem withreduction in coverage due to insufficient transmission power of theterminal or with determination of the base station to which a requestfor a transmission resource is to be made and uplink data is to betransmitted when transmitting uplink data (link selection problem).Therefore, in order to avoid the above problem, the terminal maytransmit uplink data to only one of the source base station and thetarget base station in the 5-2^(nd) embodiment.

Therefore, the terminal may make a request for scheduling to only one ofthe source base station and the target base station, may report the sizeof data to be transmitted in the PDCP entity {e.g., a buffer statusreport (BSR)} to only one of the source base station and the target basestation, may receive an uplink transmission resource, and may thentransmit uplink data to only one base station. In addition, even if theterminal receives a handover command message from the source basestation, the terminal may continue transmission and reception of datadue to HARQ retransmission in order to prevent loss of data, and thusmay not initialize a MAC entity of the first bearer.

In the 5-2^(nd) embodiment, in step 1002, the terminal 1020 may continueto transmit and receive data to and from the source base station (uplinkdata transmission and downlink data transmission) through the protocolentities of the first bearer even when performing a procedure of randomaccess to the target base station 1010 indicated by the handover commandmessage through the protocol entities of the second bearer.

In the 5-2^(nd) embodiment, in step 1003, the terminal 1020 may stoptransmitting uplink data to the source base station through the protocolentities 1022 of the first bearer if a second condition is satisfied,may transmit uplink data to the target base station through the protocolentities 1021 of the second bearer, and may continue receiving downlinkdata from the source base station and the target base station throughthe protocol entities of the first bearer and the second bearer. In thiscase, the terminal may perform an RLC re-establishment procedure for thetransmission RLC entity among the protocol entities when stoppingtransmitting uplink data to the source base station through the protocolentities 1022 of the first bearer. In addition, the PDCP entity 1021 ofthe second bearer may continue to perform the seamless transmission andreception of data to and from the target base station using informationsuch as transmission/reception data, serial number information, headercompression and decompression contexts, or the like, which are stored inthe PDCP entity 1022 of the first bearer. The second condition mayinclude one of the following conditions.

-   -   When the terminal performs a procedure of random access to the        target base station through the entities 1021 of the second        bearer and receives a random access response    -   when the terminal performs a procedure of random access to the        target base station through the entities of the second bearer,        receives a random access response, and configures and transmits        a handover completion message to the target base station    -   when the terminal completes a procedure of random access to the        target base station through the entities of the second bearer        and initially transmits data using a PUCCH or PUSCH uplink        transmission resource    -   when the base station configures a separate timer for the        terminal through an RRC message and the timer expires    -   the timer may start when the terminal receives a handover        command message from the source base station, when the terminal        starts random access to the target base station (when        transmitting a preamble), when the terminal receives a random        access response from the target base station, when the terminal        transmits a handover completion message to the target base        station, or when the terminal initially transmits data using a        PUCCH or PUSCH uplink transmission resource.    -   when the terminal performs a procedure of random access to the        target base station through the entities of the second bearer,        receives a random access response, and configures and transmits        a handover completion message to the target base station and        when the successful transmission of the handover completion        message is identified by a MAC entity (HARQ ACK) or an RLC        entity (RLC ACK)    -   when the terminal performs a procedure of random access to the        target base station through the entities of the second bearer,        receives a random access response, configures and transmits a        handover completion message to the target base station, and        receives an uplink transmission resource that is initially        allocated by the target base station, or receives an initial        indication of an uplink transmission resource    -   when the terminal receives, from the target base station, an        indication {e.g., an RRC message (e.g., an “RRCReconfiguration”        message), a MAC CE, an RLC control PDU, or a PDCP control PDU}        to release the connection with the source base station and to        switch the uplink to the target base station    -   when the terminal receives, from the source base station, an        indication {e.g., an RRC message (e.g., an “RRCReconfiguration”        message), a MAC CE, an RLC control PDU, or a PDCP control PDU}        to release the connection with the source base station and to        switch the uplink to the target base station    -   when the terminal successfully completes a procedure of random        access to the target base station through the entities of the        second bearer, and receives a first uplink transmission resource        allocated by the target base station, or receives an indication        of a first uplink transmission resource    -   for example, more specifically, if the terminal is instructed to        perform random access to the target base station by receiving a        handover command message from the source base station, and if        the indicated random access is contention-free random access        (CFRA) {for example, if a predetermined preamble or a UE cell        identifier (e.g., C-RNTI) is allocated}        -   the terminal may determine that the random access procedure            is successfully completed when the terminal transmits a            predetermined preamble to the cell of the target base            station and receives a random access response (RAR) message.            Accordingly, when the terminal receives a first uplink            transmission resource allocated, included, or indicated in            the random access response message, the terminal may            determine that the second condition is satisfied.    -   If the terminal is instructed to perform random access to the        target base station by receiving a handover command message from        the source base station, and if the indicated random access is        contention-based random access (CBRA) {for example, if a        predetermined preamble or a UE cell identifier (e.g., C-RNTI) is        not allocated)        -   the terminal may determine that the procedure of random            access to the target base station is successfully completed            when the terminal transmits a preamble (e.g., an arbitrary            preamble) to the cell of the target base station, receives a            random access response (RAR) message, transmits message 3            (e.g., a handover completion message) using the uplink            transmission resource allocated, included, or indicated in            the random access response message, and receives a MAC CE            (contention resolution MAC CE) indicating that the            contention has been resolved from the target base station.            Accordingly, thereafter, when the terminal monitors a PDCCH            and initially receives an uplink transmission resource            through the PDCCH corresponding to the C-RNTI of the            terminal or initially receives an indication thereof, the            terminal may determine that the second condition is            satisfied. As another method, if the size of the uplink            transmission resource allocated by the random access            response message is sufficient enough for the terminal to            further transmit uplink data, as well as message 3, the            terminal may determine that an initial uplink transmission            resource has been received, thereby determining that the            second condition is satisfied.    -   In the case where a handover method that does not require a        random access procedure (RACH-less handover) is indicated by the        handover command message received by the terminal    -   if the handover command message includes an uplink transmission        resource to the target base station        -   the terminal may determine that the random access procedure            is successfully completed when the terminal transmits            message 3 (e.g., a handover completion message or an            “RRCReconfigurationComplete” message) using the uplink            transmission resource of the target base station and            receives a UE identity confirmation MAC CE from the base            station, thereby determining that the second condition is            satisfied. As another method, when the terminal receives a            first uplink transmission resource through a PDCCH            corresponding to a C-RNTI of the terminal by monitoring the            PDCCH after a random access procedure is successfully            completed, the terminal may determine that the second            condition is satisfied.    -   In the case where the handover command message does not include        an uplink transmission resource for the target base station        -   when the terminal receives an uplink transmission resource            through a PDCCH corresponding to a C-RNTI of the terminal by            monitoring the PDCCH for the target base station (or cell),            or when the terminal transmits message 3 (e.g., a handover            completion message or an “RRCReconfigurationComplete”            message) using the uplink transmission resource and receives            a UE identity confirmation MAC CE from the base station, the            terminal may determine that the random access procedure is            successfully completed, thereby determining that the second            condition is satisfied. As another method, when the terminal            receives a first uplink transmission resource through a            PDCCH corresponding to a C-RNTI of the terminal by            monitoring the PDCCH after a random access procedure is            successfully completed, the terminal may determine that the            second condition is satisfied.

In the case where the terminal is able to continue to receive downlinkdata from the source base station and the target base station throughthe protocol entities of the first bearer and the second bearer, inorder to facilitate reception of downlink data from the source basestation (or the target base station) or facilitate the source basestation (or the target base station) to transmit downlink data, for AMbearers, an RLC status report other than data is allowed to becontinuously transmitted to the source base station (or the target basestation) in uplink through the protocol entities of the first bearer (orthe second bearer). This is due to the fact that if an RLC status reportof successful delivery is not indicated (that is, if the RLC statusreport is not received) after the AM bearers transmit data to atransmitting end, the AM bearers are unable to continue to transmit datathereafter.

In addition, in order to facilitate reception of downlink data from thesource base station (or the target base station) or facilitate thesource base station (or the target base station) to transmit downlinkdata, continuous transmission of HARQ ACK or HARQ NACK information orPDCP control data (for example, a PDCP status report or ROHC feedbackinformation) may be allowed through the protocol entities of the firstbearer (or the second bearer).

Specifically, in the case where handover corresponding to the 5-2^(nd)embodiment of the disclosure is indicated when receiving handovercommand message, the terminal may perform scheduling request through thefirst protocol entity until the second condition is satisfied, maytransmit a buffer status report to the source base station, may receivean uplink transmission resource, may transmit uplink data, and mayreceive downlink data from the source base station.

However, if the second condition is satisfied, the terminal may nolonger transmit data to the source base station, may switch the uplinkto perform scheduling request through the second protocol entity, maytransmit a buffer status report to the target base station, may receivean uplink transmission resource, and may transmit uplink data to thetarget base station.

However, the terminal may continue to receive downlink data from thesource base station, and may continue to transmit HARQ ACK, HARQ NACK,an RLC status report, or PDCP control data (for example, a PDCP statusreport or ROHC feedback information) corresponding to the downlink data.The terminal may also receive downlink data from the target base stationwhen the second condition is satisfied.

In the 5-2^(nd) embodiment, in step 1004, the terminal 1020 may stopreceiving downlink data from the source base station 1005 through theprotocol entities 1022 of the first bearer if a first condition issatisfied. In addition, the PDCP entity 1021 of the second bearer maycontinue to perform the seamless transmission and reception of data toand from the target base station using information such astransmission/reception data, serial number information, headercompression and decompression contexts, or the like, which are stored inthe PDCP entity 1022 of the first bearer. The first condition mayinclude one of the following conditions.

-   -   When the terminal performs a procedure of random access to the        target base station through the entities 1021 of the second        bearer and receives a random access response    -   when the terminal performs a procedure of random access to the        target base station through the entities of the second bearer,        receives a random access response, and configures and transmits        a handover completion message to the target base station    -   when the terminal completes a procedure of random access to the        target base station through the entities of the second bearer        and initially transmits data using a PUCCH or PUSCH uplink        transmission resource    -   when the base station configures a separate timer for the        terminal through an RRC message and the timer expires    -   the timer may start when the terminal receives a handover        command message from the source base station, when the terminal        starts random access to the target base station (when        transmitting a preamble), when the terminal receives a random        access response from the target base station, when the terminal        transmits a handover completion message to the target base        station, or when the terminal initially transmits data using a        PUCCH or PUSCH uplink transmission resource.    -   when the terminal performs a procedure of random access to the        target base station through the entities of the second bearer,        receives a random access response, and configures and transmits        a handover completion message to the target base station and        when the successful transmission of the handover completion        message is identified by a MAC entity (HARQ ACK) or an RLC        entity (RLC ACK)    -   when the terminal performs a procedure of random access to the        target base station through the entities of the second bearer,        receives a random access response, configures and transmits a        handover completion message to the target base station, and        receives an uplink transmission resource that is initially        allocated by the target base station, or receives an initial        indication of an uplink transmission resource    -   the source base station may determine the time to stop        transmitting the downlink data to the terminal or the time to        release the connection with the terminal, based on a        predetermined method when performing the efficient handover        proposed in the disclosure. For example, the predetermined        method may include at least one of when a predetermined timer        expires (the timer may start after indication of handover) or        when an indication indicating that the terminal has successfully        performed the handover to the target base station is received        from the target base station.

In addition, if no downlink data is received from the source basestation for a predetermined period of time, the terminal may determinethat the connection with the source base station is released, and mayrelease the connection.

-   -   When the terminal receives, from the target base station, an        indication {e.g., an RRC message (e.g., an “RRCReconfiguration”        message), a MAC CE, an RLC control PDU, or a PDCP control PDU}        to release the connection with the source base station    -   when the terminal receives, from the source base station, an        indication {e.g., an RRC message (e.g., an “RRCReconfiguration”        message), a MAC CE, an RLC control PDU, or a PDCP control PDU}        to release the connection with the source base station    -   when the terminal fails to receive downlink data from the source        base station for a predetermined period of time    -   when the terminal successfully completes a procedure of random        access to the target base station through the entities of the        second bearer, and receives a first uplink transmission resource        allocated by the target base station, or receives an indication        of a first uplink transmission resource    -   for example, more specifically, if the terminal is instructed to        perform random access to the target base station by receiving a        handover command message from the source base station, and if        the indicated random access is contention-free random access        (CFRA) {for example, if a predetermined preamble or a UE cell        identifier (e.g., C-RNTI) is allocated}        -   the terminal may determine that the random access procedure            is successfully completed when the terminal transmits a            predetermined preamble to the cell of the target base            station and receives a random access response (RAR) message.            Accordingly, when the terminal receives a first uplink            transmission resource allocated, included, or indicated in            the random access response message, the terminal may            determine that the first condition is satisfied.    -   If the terminal is instructed to perform random access to the        target base station by receiving a handover command message from        the source base station, and if the indicated random access is        contention-based random access (CBRA) {for example, if a        predetermined preamble or a UE cell identifier (e.g., C-RNTI) is        not allocated)        -   the terminal may determine that the procedure of random            access to the target base station is successfully completed            when the terminal transmits a preamble (e.g., an arbitrary            preamble) to the cell of the target base station, receives a            random access response (RAR) message, transmits message 3            (e.g., a handover completion message) using the uplink            transmission resource allocated, included, or indicated in            the random access response message, and receives a MAC CE            (contention resolution MAC CE) indicating that the            contention has been resolved from the target base station.            Accordingly, thereafter, when the terminal monitors a PDCCH            and initially receives an uplink transmission resource            through the PDCCH corresponding to the C-RNTI of the            terminal or initially receives an indication thereof, the            terminal may determine that the first condition is            satisfied. As another method, if the size of the uplink            transmission resource allocated by the random access            response message is sufficient enough for the terminal to            further transmit uplink data, as well as message 3, the            terminal may determine that an initial uplink transmission            resource has been received, thereby determining that the            first condition is satisfied.    -   In the case where a handover method that does not require a        random access procedure (RACH-less handover) is indicated by the        handover command message received by the terminal    -   if the handover command message includes an uplink transmission        resource to the target base station        -   the terminal may determine that the random access procedure            is successfully completed when the terminal transmits            message 3 (e.g., a handover completion message or an            “RRCReconfigurationComplete” message) using the uplink            transmission resource of the target base station and            receives a UE identity confirmation MAC CE from the base            station, thereby determining that the first condition is            satisfied. As another method, when the terminal receives a            first uplink transmission resource through a PDCCH            corresponding to a C-RNTI of the terminal by monitoring the            PDCCH after a random access procedure is successfully            completed, the terminal may determine that the first            condition is satisfied.    -   In the case where the handover command message does not include        an uplink transmission resource for the target base station        -   when the terminal receives an uplink transmission resource            through a PDCCH corresponding to a C-RNTI of the terminal by            monitoring the PDCCH for the target base station (or cell),            or when the terminal transmits message 3 (e.g., a handover            completion message or an “RRCReconfigurationComplete”            message) using the uplink transmission resource and receives            a UE identity confirmation MAC CE from the base station, the            terminal may determine that the random access procedure is            successfully completed, thereby determining that the first            condition is satisfied. As another method, when the terminal            receives a first uplink transmission resource through a            PDCCH corresponding to a C-RNTI of the terminal by            monitoring the PDCCH after a random access procedure is            successfully completed, the terminal may determine that the            first condition is satisfied.

Meanwhile, the indicators for the embodiments proposed by the disclosuremay be defined. Thus, when transmitting the handover command message tothe terminal (620) in FIG. 6 as described above, the base station mayindicate, to the terminal, the handover procedure corresponding to aspecific embodiment to be triggered through the handover command message(e.g., an RRCReconfiguration message). In addition, the terminal mayperform a handover procedure according to the handover method indicatedby the handover command message, thereby performing handover to thetarget base station while minimizing data interruption time.

As another method, the indicators for the embodiments proposed by thedisclosure may be defined for respective bearers. Thus, the base stationmay indicate more specifically the embodiment and the bearer to whichhandover is applied using the handover command message. For example, theembodiments may be applied only to an AM bearer in which an RLC entityis driven in an AM, or may be extendedly applied to a UM bearer in whichan RLC entity is driven in a UM.

In addition, it is assumed that the embodiments proposed in thedisclosure are applied to the DRBs. However, if necessary (for example,when the terminal maintains an SRB for the source base station and failsto perform handover to the target base station, thereby reporting ahandover failure message or recovering the same through the SRB for thesource base station), the embodiments proposed in the disclosure may beextendedly applied to the SRB.

In the embodiments, when the terminal transmits and receives data to andfrom the source base station through the protocol entities of the firstbearer, and when the terminal transmits and receives data to and fromthe target base station through the protocol entities of the secondbearer, the MAC entity of the first bearer and the MAC entity of thesecond bearer may reduce battery consumption of the terminal usingseparate discontinuous reception (DRX) periods. That is, the terminalmay continue to apply the DRX period of the MAC entity, which has beenapplied when transmitting and receiving data through the protocolentities of the first bearer, even after receiving a handover commandmessage, and may stop the DRX according to the first condition or thesecond condition in the disclosure. In addition, the terminal mayseparately apply the DRX period to the MAC entity of the second beareraccording to the indication of the target base station.

In addition, in the disclosure, the operation in which the terminalstops uplink transmission to the source base station through theprotocol entities of the first bearer and stops downlink transmissionfrom the source base station refers to the operation in which theterminal re-establishes, initializes, or releases the protocol entitiesof the first bearer (PHY entities, MAC entities, RLC entities, or PDCPentities).

For the convenience of description, it has been described that theterminal has the first bearer for the source base station or the secondbearer for the target base station in the above embodiments, and theembodiments may be readily extended and applied, in the same manner, tothe case where the terminal has a plurality of first bearers for thesource base station or a plurality of second bearers for the target basestation.

In addition, the embodiments of the disclosure may be readily extendedand applied, in the same manner, to the case where a plurality ofbearers is configured for a plurality of target base stations. Forexample, the terminal may configure second bearers while performing ahandover procedure to a first target base station, and if the handoveris unsuccessful, the terminal may configure second bearers whileperforming a handover procedure to a second target base station. Asdescribed above, the terminal may search for and determine a cell thatsatisfies a predetermined condition (for example, a predetermined signalstrength or more) among the plurality of target base stations, therebyperforming the handover procedure.

FIG. 11 is a diagram showing the structure of an efficient PDCP entitythat may be applied to embodiments.

The disclosure proposes the structure of an efficient PDCP entity asshown in FIG. 11 . The structure of the PDCP entity shown in FIG. 11 maybe applied to the second embodiment, the third embodiment, the fourthembodiment, or the fifth embodiment of the efficient handover method forminimizing data interruption time proposed in the disclosure.

In FIG. 11 , the terminal 1120 may transmit and receive data to and froma source base station 1105 through protocol entities of a first bearer,and may transmit and receive data to and from a target base station 1110through protocol entities of a second bearer at the same time.

Although a PDCP entity of the first bearer and a PDCP entity of thesecond bearer are configured in the terminal, respectively, the PDCPentity of the first bearer and the PDCP entity of the second bearer maylogically operate as a single PDCP entity as shown in FIG. 11 .Specifically, the single PDCP entity may be implemented to have an upperPDCP entity 1123 and two lower PDCP entities 1121 and 1122 for thesource base station and the target base station, respectively, accordingto functions of the PDCP entity.

The upper transmitting PDCP entity 1123 may serve to assign PDCP serialnumbers to data received from an upper layer entity. In addition, theupper transmitting PDCP entity 1123 may perform header compression. Inaddition, the lower transmitting PDCP entities 1121 and 1122 for thesource base station and the target base station may apply an integrityprotection procedure to the PDCP header and data (PDCP SDUs), in thecase where integrity protection is configured using separate securitykeys set for the source base station and the target base station, mayapply a ciphering procedure to the PDCP header and data, and maytransmit the same to a transmitting RLC entity of the first bearer or atransmitting RLC entity of the second bearer.

In order to accelerate a data processing rate, the lower transmittingPDCP entities 1121 and 1122 may perform parallel processing in whichheader compression, integrity protection, and/or ciphering proceduresare performed in parallel, and may perform the integrity protection orciphering procedure using different security keys. In addition, theintegrity protection or ciphering procedure may be performed ondifferent data in logically one transmitting PDCP entity using differentsecurity keys or security algorithms.

The upper receiving PDCP entity 1123 may perform a duplicate detectionfunction on the data received from lower layer entities, based on PDCPserial numbers. In addition, the upper receiving PDCP entity 1123 maysort the received data in ascending order of the PDCP serial numbers,and may transmit the same to the upper layer in sequence. In addition,the upper receiving PDCP entity 1123 may perform header decompression.

Further, the lower receiving PDCP entities 1121 and 1122 for respectiveones of the source base station and the target base station may apply anintegrity verification procedure to the PDCP header and data (PDCPSDUs), in the case where integrity protection is configured usingseparate security keys set for the source base station and the targetbase station, may apply a deciphering procedure to the PDCP header anddata, and may transmit the same to the upper receiving PDCP entity,thereby processing the data.

In order to reduce unnecessary integrity verification or decipheringprocedures, the lower receiving PDCP entities may perform a procedure ofdiscarding data outside the window and discarding duplicate data, basedon the PDCP serial numbers, and may perform the integrity verificationor deciphering procedure only on the valid data inside the window.

In order to accelerate a data processing rate, the lower transmittingPDCP entities may perform parallel processing in which headercompression, integrity protection, and ciphering procedures areperformed in parallel, based on the PDCP serial numbers, and may performthe integrity protection verification or deciphering procedure usingdifferent security keys. In addition, the integrity protection orciphering procedure may be performed on different data in logically onetransmitting PDCP entity using different security keys or securityalgorithms. Further, the lower receiving PDCP entities may perform anout-of-sequence deciphering or integrity verification procedure for eachpiece of data received regardless of the sequence of the PDCP serialnumbers.

The entities of the first bearer (or a first RLC entity) and theentities of the second bearer (or a second RLC entity) for a single PDCPentity are distinguished by considering that they are connected todifferent MAC entities, by allowing them to have different logicalchannel identifiers, by considering that they are different RLC entitiesconnected to different MAC entities, or based on that they use differentciphering keys, and uplink data and downlink data may be ciphered ordeciphered using different security keys, and may be compressed ordecompressed using different compression protocol contexts.

When the fourth or fifth embodiment proposed in the disclosure may beindicated to the terminal through a handover command message. At thistime, if a fourth condition below is satisfied for each bearer, theterminal may convert, change, or reset the PDCP entity, which is usedfor each bearer before the handover command message is received, intothe efficient PDCP entity proposed in FIG. 11 , and may apply the same.The fourth condition may be one or more of the following conditions.

-   -   In the case where the terminal receives, from the source base        station, a handover command message that indicates to perform        handover using the fourth or fifth embodiment proposed in the        disclosure or indicates the method of applying the efficient        PDCP entity structure proposed in FIG. 11    -   when the terminal performs a procedure of random access to the        target base station through the entities 1021 of the second        bearer and receives a random access response    -   when the terminal performs a procedure of random access to the        target base station through the entities of the second bearer,        receives a random access response, and configures and transmits        a handover completion message to the target base station    -   when the terminal completes a procedure of random access to the        target base station through the entities of the second bearer        and initially transmits data using a PUCCH or PUSCH uplink        transmission resource    -   when the base station configures a separate timer for the        terminal through an RRC message and the timer expires    -   the timer may start when the terminal receives a handover        command message from the source base station, when the terminal        starts random access to the target base station (when        transmitting a preamble), when the terminal receives a random        access response from the target base station, when the terminal        transmits a handover completion message to the target base        station, or when the terminal initially transmits data using a        PUCCH or PUSCH uplink transmission resource.    -   when the terminal performs a procedure of random access to the        target base station through the entities of the second bearer,        receives a random access response, and configures and transmits        a handover completion message to the target base station and        when the successful transmission of the handover completion        message is identified by a MAC entity (HARQ ACK) or an RLC        entity (RLC ACK)    -   when the terminal performs a procedure of random access to the        target base station through the entities of the second bearer,        receives a random access response, configures and transmits a        handover completion message to the target base station, and        receives an uplink transmission resource that is initially        allocated by the target base station, or receives an initial        indication of an uplink transmission resource    -   when the terminal successfully completes a procedure of random        access to the target base station through the entities of the        second bearer, and receives a first uplink transmission resource        allocated by the target base station, or receives an indication        of a first uplink transmission resource    -   for example, more specifically, if the terminal is instructed to        perform random access to the target base station by receiving a        handover command message from the source base station, and if        the indicated random access is contention-free random access        (CFRA) {for example, if a predetermined preamble or a UE cell        identifier (e.g., C-RNTI) is allocated}        -   the terminal may determine that the random access procedure            is successfully completed when the terminal transmits a            predetermined preamble to the cell of the target base            station and receives a random access response (RAR) message.            Accordingly, when the terminal receives a first uplink            transmission resource allocated, included, or indicated in            the random access response message, the terminal may            determine that the fourth condition is satisfied.    -   If the terminal is instructed to perform random access to the        target base station by receiving a handover command message from        the source base station, and if the indicated random access is        contention-based random access (CBRA) {for example, if a        predetermined preamble or a UE cell identifier (e.g., C-RNTI) is        not allocated)        -   the terminal may determine that the procedure of random            access to the target base station is successfully completed            when the terminal transmits a preamble (e.g., an arbitrary            preamble) to the cell of the target base station, receives a            random access response (RAR) message, transmits message 3            (e.g., a handover completion message) using the uplink            transmission resource allocated, included, or indicated in            the random access response message, and receives a MAC CE            (contention resolution MAC CE) indicating that the            contention has been resolved from the target base station.            Accordingly, thereafter, when the terminal monitors a PDCCH            and initially receives an uplink transmission resource            through the PDCCH corresponding to the C-RNTI of the            terminal or initially receives an indication thereof, the            terminal may determine that the fourth condition is            satisfied. As another method, if the size of the uplink            transmission resource allocated by the random access            response message is sufficient enough for the terminal to            further transmit uplink data, as well as message 3, the            terminal may determine that an initial uplink transmission            resource has been received, thereby determining that the            fourth condition is satisfied.    -   In the case where a handover method that does not require a        random access procedure (RACH-less handover) is indicated by the        handover command message received by the terminal    -   if the handover command message includes an uplink transmission        resource to the target base station        -   the terminal may determine that the random access procedure            is successfully completed when the terminal transmits            message 3 (e.g., a handover completion message or an            “RRCReconfigurationComplete” message) using the uplink            transmission resource of the target base station and            receives a UE identity confirmation MAC CE from the base            station, thereby determining that the fourth condition is            satisfied. As another method, when the terminal receives a            first uplink transmission resource through a PDCCH            corresponding to a C-RNTI of the terminal by monitoring the            PDCCH after a random access procedure is successfully            completed, the terminal may determine that the fourth            condition is satisfied.    -   In the case where the handover command message does not include        an uplink transmission resource for the target base station        -   when the terminal receives an uplink transmission resource            through a PDCCH corresponding to a C-RNTI of the terminal by            monitoring the PDCCH for the target base station (or cell),            or when the terminal transmits message 3 (e.g., a handover            completion message or an “RRCReconfigurationComplete”            message) using the uplink transmission resource and receives            a UE identity confirmation MAC CE from the base station, the            terminal may determine that the random access procedure is            successfully completed, thereby determining that the fourth            condition is satisfied. As another method, when the terminal            receives a first uplink transmission resource through a            PDCCH corresponding to a C-RNTI of the terminal by            monitoring the PDCCH after a random access procedure is            successfully completed, the terminal may determine that the            fourth condition is satisfied.

In addition, when the fourth or fifth embodiment proposed in thedisclosure is indicated to the terminal by a handover command message,if the fourth condition is satisfied for each bearer, the terminal mayconvert, change, or reset the PDCP entity, which is used for each bearerbefore the handover command message is received, into the efficient PDCPentity proposed in FIG. 11 , and may apply the same.

Further, if a fifth condition is satisfied, the terminal may stopreceiving downlink data from the source base station, and may reconvert,re-change, or reset the structure of the PDCP entity, which has beenconverted, changed, or reset into the efficient PDCP entity proposed inFIG. 11 , into the PDCP entity used for each bearer before the handovercommand message is received. The fifth condition may be one or more ofthe following conditions.

-   -   When the terminal performs a procedure of random access to the        target base station through the entities 1021 of the second        bearer and receives a random access response    -   when the terminal performs a procedure of random access to the        target base station through the entities of the second bearer,        receives a random access response, and configures and transmits        a handover completion message to the target base station    -   when the terminal completes a procedure of random access to the        target base station through the entities of the second bearer        and initially transmits data using a PUCCH or PUSCH uplink        transmission resource    -   when the base station configures a separate timer for the        terminal through an RRC message and the timer expires    -   the timer may start when the terminal receives a handover        command message from the source base station, when the terminal        starts random access to the target base station (when        transmitting a preamble), when the terminal receives a random        access response from the target base station, when the terminal        transmits a handover completion message to the target base        station, or when the terminal initially transmits data using a        PUCCH or PUSCH uplink transmission resource.    -   when the terminal performs a procedure of random access to the        target base station through the entities of the second bearer,        receives a random access response, and configures and transmits        a handover completion message to the target base station and        when the successful transmission of the handover completion        message is identified by a MAC entity (HARQ ACK) or an RLC        entity (RLC ACK)    -   when the terminal performs a procedure of random access to the        target base station through the entities of the second bearer,        receives a random access response, configures and transmits a        handover completion message to the target base station, and        receives an uplink transmission resource that is initially        allocated by the target base station, or receives an initial        indication of an uplink transmission resource    -   when the terminal receives, from the target base station, an        indication {e.g., an RRC message (e.g., an “RRCReconfiguration”        message), a MAC CE, an RLC control PDU, or a PDCP control PDU}        to release the connection with the source base station    -   when the terminal receives, from the source base station, an        indication {e.g., an RRC message (e.g., an “RRCReconfiguration”        message), a MAC CE, an RLC control PDU, or a PDCP control PDU}        to release the connection with the source base station    -   when the terminal fails to receive downlink data from the source        base station for a predetermined period of time    -   when the terminal successfully completes a procedure of random        access to the target base station through the entities of the        second bearer, and receives a first uplink transmission resource        allocated by the target base station, or receives an indication        of a first uplink transmission resource    -   for example, more specifically, if the terminal is instructed to        perform random access to the target base station by receiving a        handover command message from the source base station, and if        the indicated random access is contention-free random access        (CFRA) {for example, if a predetermined preamble or a UE cell        identifier (e.g., C-RNTI) is allocated}        -   the terminal may determine that the random access procedure            is successfully completed when the terminal transmits a            predetermined preamble to the cell of the target base            station and receives a random access response (RAR) message.            Accordingly, when the terminal receives a first uplink            transmission resource allocated, included, or indicated in            the random access response message, the terminal may            determine that the fifth condition is satisfied.    -   If the terminal is instructed to perform random access to the        target base station by receiving a handover command message from        the source base station, and if the indicated random access is        contention-based random access (CBRA) {for example, if a        predetermined preamble or a UE cell identifier (e.g., C-RNTI) is        not allocated)        -   the terminal may determine that the procedure of random            access to the target base station is successfully completed            when the terminal transmits a preamble (e.g., an arbitrary            preamble) to the cell of the target base station, receives a            random access response (RAR) message, transmits message 3            (e.g., a handover completion message) using the uplink            transmission resource allocated, included, or indicated in            the random access response message, and receives a MAC CE            (contention resolution MAC CE) indicating that the            contention has been resolved from the target base station.            Accordingly, thereafter, when the terminal monitors a PDCCH            and initially receives an uplink transmission resource            through the PDCCH corresponding to the C-RNTI of the            terminal or initially receives an indication thereof, the            terminal may determine that the fifth condition is            satisfied. As another method, if the size of the uplink            transmission resource allocated by the random access            response message is sufficient enough for the terminal to            further transmit uplink data, as well as message 3, the            terminal may determine that an initial uplink transmission            resource has been received, thereby determining that the            fifth condition is satisfied.    -   In the case where a handover method that does not require a        random access procedure (RACH-less handover) is indicated by the        handover command message received by the terminal    -   if the handover command message includes an uplink transmission        resource to the target base station        -   the terminal may determine that the random access procedure            is successfully completed when the terminal transmits            message 3 (e.g., a handover completion message or an            “RRCReconfigurationComplete” message) using the uplink            transmission resource of the target base station and            receives a UE identity confirmation MAC CE from the base            station, thereby determining that the fifth condition is            satisfied. As another method, when the terminal receives a            first uplink transmission resource through a PDCCH            corresponding to a C-RNTI of the terminal by monitoring the            PDCCH after a random access procedure is successfully            completed, the terminal may determine that the fifth            condition is satisfied.    -   In the case where the handover command message does not include        an uplink transmission resource for the target base station        -   when the terminal receives an uplink transmission resource            through a PDCCH corresponding to a C-RNTI of the terminal by            monitoring the PDCCH for the target base station (or cell),            or when the terminal transmits message 3 (e.g., a handover            completion message or an “RRCReconfigurationComplete”            message) using the uplink transmission resource and receives            a UE identity confirmation MAC CE from the base station, the            terminal may determine that the random access procedure is            successfully completed, thereby determining that the fifth            condition is satisfied. As another method, when the terminal            receives a first uplink transmission resource through a            PDCCH corresponding to a C-RNTI of the terminal by            monitoring the PDCCH after a random access procedure is            successfully completed, the terminal may determine that the            fifth condition is satisfied.

As another method, if the fifth condition is satisfied, the terminal maystop receiving downlink data from the source base station, may releasethe RLC entity or the MAC entity, which is the first protocol entity,for each bearer, and may apply the structure of the efficient PDCPentity proposed in FIG. 11 to respective bearers, thereby using thesame.

FIG. 12 is a display illustrating the operation of a terminal that maybe applied to the embodiments proposed in the disclosure.

Referring to FIG. 12 , a terminal may receive a handover command message(1205). The terminal may establish protocol entities of a second bearerfor a target base station indicated by the message (1210).

Then, the terminal may perform a procedure of random access to thetarget base station through the established protocol entities (1215).The terminal may continue to transmit and receive data to and from asource base station (uplink data transmission and downlink datatransmission) through protocol entities of a first bearer whileperforming the random access procedure (1220).

The terminal may identify whether or not a second condition is satisfied(1225), and if the second condition is satisfied, the terminal may stoptransmitting uplink data to the source base station through the protocolentities of the first bearer, and may transmit uplink data to the targetbase station through the protocol entities of the second bearer whilecontinuing to receive downlink data from the source base station and thetarget base station through the protocol entities of the first bearerand the second bearer (1230). In addition, the PDCP entity of the secondbearer may continue to perform the seamless transmission and receptionof data to and from the target base station using information such astransmission/reception data, serial number information, headercompression and decompression contexts, or the like, which are stored inthe PDCP entity of the first bearer. The second condition may be one ofthe following conditions.

-   -   When the terminal performs a procedure of random access to the        target base station through the entities of the second bearer        and receives a random access response    -   when the terminal performs a procedure of random access to the        target base station through the entities of the second bearer,        receives a random access response, and configures and transmits        a handover completion message to the target base station    -   when the terminal completes a procedure of random access to the        target base station through the entities of the second bearer        and initially transmits data using a PUCCH or PUSCH uplink        transmission resource    -   when the base station configures a separate timer for the        terminal through an RRC message and the timer expires    -   the timer may start when the terminal receives a handover        command message from the source base station, when the terminal        starts random access to the target base station (when        transmitting a preamble), when the terminal receives a random        access response from the target base station, when the terminal        transmits a handover completion message to the target base        station, or when the terminal initially transmits data using a        PUCCH or PUSCH uplink transmission resource.    -   when the terminal performs a procedure of random access to the        target base station through the entities of the second bearer,        receives a random access response, and configures and transmits        a handover completion message to the target base station and        when the successful transmission of the handover completion        message is identified by a MAC entity (HARQ ACK) or an RLC        entity (RLC ACK)    -   when the terminal performs a procedure of random access to the        target base station through the entities of the second bearer,        receives a random access response, configures and transmits a        handover completion message to the target base station, and        receives an uplink transmission resource that is initially        allocated by the target base station, or receives an initial        indication of an uplink transmission resource    -   when the terminal receives, from the target base station, an        indication {e.g., an RRC message (e.g., an “RRCReconfiguration”        message), a MAC CE, an RLC control PDU, or a PDCP control PDU}        to release the connection with the source base station and to        switch the uplink to the target base station    -   when the terminal receives, from the source base station, an        indication {e.g., an RRC message (e.g., an “RRCReconfiguration”        message), a MAC CE, an RLC control PDU, or a PDCP control PDU}        to release the connection with the source base station and to        switch the uplink to the target base station    -   in the case where the terminal receives, from the source base        station, a handover command message that indicates to perform        handover using the fourth or fifth embodiment proposed in the        disclosure or indicates the method of applying an efficient PDCP        entity structure proposed in FIG. 11    -   when the terminal successfully completes a procedure of random        access to the target base station through the entities of the        second bearer, and receives a first uplink transmission resource        allocated by the target base station, or receives an indication        of a first uplink transmission resource    -   for example, more specifically, if the terminal is instructed to        perform random access to the target base station by receiving a        handover command message from the source base station, and if        the indicated random access is contention-free random access        (CFRA) {for example, if a predetermined preamble or a UE cell        identifier (e.g., C-RNTI) is allocated}        -   the terminal may determine that the random access procedure            is successfully completed when the terminal transmits a            predetermined preamble to the cell of the target base            station and receives a random access response (RAR) message.            Accordingly, when the terminal receives a first uplink            transmission resource allocated, included, or indicated in            the random access response message, the terminal may            determine that the second condition is satisfied.    -   If the terminal is instructed to perform random access to the        target base station by receiving a handover command message from        the source base station, and if the indicated random access is        contention-based random access (CBRA) {for example, if a        predetermined preamble or a UE cell identifier (e.g., C-RNTI) is        not allocated)        -   the terminal may determine that the procedure of random            access to the target base station is successfully completed            when the terminal transmits a preamble (e.g., an arbitrary            preamble) to the cell of the target base station, receives a            random access response (RAR) message, transmits message 3            (e.g., a handover completion message) using the uplink            transmission resource allocated, included, or indicated in            the random access response message, and receives a MAC CE            (contention resolution MAC CE) indicating that the            contention has been resolved from the target base station.            Accordingly, thereafter, when the terminal monitors a PDCCH            and initially receives an uplink transmission resource            through the PDCCH corresponding to the C-RNTI of the            terminal or initially receives an indication thereof, the            terminal may determine that the second condition is            satisfied. As another method, if the size of the uplink            transmission resource allocated by the random access            response message is sufficient enough for the terminal to            further transmit uplink data, as well as message 3, the            terminal may determine that an initial uplink transmission            resource has been received, thereby determining that the            second condition is satisfied.    -   In the case where a handover method that does not require a        random access procedure (RACH-less handover) is indicated by the        handover command message received by the terminal    -   if the handover command message includes an uplink transmission        resource to the target base station        -   the terminal may determine that the random access procedure            is successfully completed when the terminal transmits            message 3 (e.g., a handover completion message or an            “RRCReconfigurationComplete” message) using the uplink            transmission resource of the target base station and            receives a UE identity confirmation MAC CE from the base            station, thereby determining that the second condition is            satisfied. As another method, when the terminal receives a            first uplink transmission resource through a PDCCH            corresponding to a C-RNTI of the terminal by monitoring the            PDCCH after a random access procedure is successfully            completed, the terminal may determine that the second            condition is satisfied.    -   In the case where the handover command message does not include        an uplink transmission resource for the target base station        -   when the terminal receives an uplink transmission resource            through a PDCCH corresponding to a C-RNTI of the terminal by            monitoring the PDCCH for the target base station (or cell),            or when the terminal transmits message 3 (e.g., a handover            completion message or an “RRCReconfigurationComplete”            message) using the uplink transmission resource and receives            a UE identity confirmation MAC CE from the base station, the            terminal may determine that the random access procedure is            successfully completed, thereby determining that the second            condition is satisfied. As another method, when the terminal            receives a first uplink transmission resource through a            PDCCH corresponding to a C-RNTI of the terminal by            monitoring the PDCCH after a random access procedure is            successfully completed, the terminal may determine that the            second condition is satisfied.

If the second condition is not satisfied in the above step, the terminalmay repeat to check the second condition while continuing the existingprocedure (that is, the procedure of transmitting and receiving data toand from the source base station and connecting to the target basestation) (1235).

Meanwhile, if a first condition is satisfied (1240), the terminal maystop receiving downlink data from the source base station through theprotocol entities of the first bearer (1245). In addition, the PDCPentity of the second bearer may continue to perform the seamlesstransmission and reception of data to and from the target base stationusing information such as transmission/reception data, serial numberinformation, header compression and decompression contexts, or the like,which are stored in the PDCP entity of the first bearer. The firstcondition may be one of the following conditions.

-   -   When the terminal performs a procedure of random access to the        target base station through the entities of the second bearer        and receives a random access response    -   when the terminal performs a procedure of random access to the        target base station through the entities of the second bearer,        receives a random access response, and configures and transmits        a handover completion message to the target base station    -   when the terminal completes a procedure of random access to the        target base station through the entities of the second bearer        and initially transmits data using a PUCCH or PUSCH uplink        transmission resource    -   when the base station configures a separate timer for the        terminal through an RRC message and the timer expires    -   the timer may start when the terminal receives a handover        command message from the source base station, when the terminal        starts random access to the target base station (when        transmitting a preamble), when the terminal receives a random        access response from the target base station, when the terminal        transmits a handover completion message to the target base        station, or when the terminal initially transmits data using a        PUCCH or PUSCH uplink transmission resource.    -   when the terminal performs a procedure of random access to the        target base station through the entities of the second bearer,        receives a random access response, and configures and transmits        a handover completion message to the target base station and        when the successful transmission of the handover completion        message is identified by a MAC entity (HARQ ACK) or an RLC        entity (RLC ACK)    -   when the terminal performs a procedure of random access to the        target base station through the entities of the second bearer,        receives a random access response, configures and transmits a        handover completion message to the target base station, and        receives an uplink transmission resource that is initially        allocated by the target base station, or receives an initial        indication of an uplink transmission resource    -   the source base station may determine the time to stop        transmitting the downlink data to the terminal or the time to        release the connection with the terminal, based on a        predetermined method when performing the efficient handover        proposed in the disclosure. For example, the predetermined        method may include at least one of when a predetermined timer        expires (the timer may start after indication of handover) or        when an indication indicating that the terminal has successfully        performed the handover to the target base station is received        from the target base station. In addition, if no downlink data        is received from the source base station for a predetermined        period of time, the terminal may determine that the connection        with the source base station is released, and may release the        connection.    -   When the terminal receives, from the target base station, an        indication {e.g., an RRC message (e.g., an “RRCReconfiguration”        message), a MAC CE, an RLC control PDU, or a PDCP control PDU}        to release the connection with the source base station    -   when the terminal receives, from the source base station, an        indication {e.g., an RRC message (e.g., an “RRCReconfiguration”        message), a MAC CE, an RLC control PDU, or a PDCP control PDU}        to release the connection with the source base station    -   when the terminal fails to receive downlink data from the source        base station for a predetermined period of time    -   when the terminal successfully completes a procedure of random        access to the target base station through the entities of the        second bearer, and receives a first uplink transmission resource        allocated by the target base station, or receives an indication        of a first uplink transmission resource    -   for example, more specifically, if the terminal is instructed to        perform random access to the target base station by receiving a        handover command message from the source base station, and if        the indicated random access is contention-free random access        (CFRA) {for example, if a predetermined preamble or a UE cell        identifier (e.g., C-RNTI) is allocated}        -   the terminal may determine that the random access procedure            is successfully completed when the terminal transmits a            predetermined preamble to the cell of the target base            station and receives a random access response (RAR) message.            Accordingly, when the terminal receives a first uplink            transmission resource allocated, included, or indicated in            the random access response message, the terminal may            determine that the first condition is satisfied.    -   If the terminal is instructed to perform random access to the        target base station by receiving a handover command message from        the source base station, and if the indicated random access is        contention-based random access (CBRA) {for example, if a        predetermined preamble or a UE cell identifier (e.g., C-RNTI) is        not allocated)        -   the terminal may determine that the procedure of random            access to the target base station is successfully completed            when the terminal transmits a preamble (e.g., an arbitrary            preamble) to the cell of the target base station, receives a            random access response (RAR) message, transmits message 3            (e.g., a handover completion message) using the uplink            transmission resource allocated, included, or indicated in            the random access response message, and receives a MAC CE            (contention resolution MAC CE) indicating that the            contention has been resolved from the target base station.            Accordingly, thereafter, when the terminal monitors a PDCCH            and initially receives an uplink transmission resource            through the PDCCH corresponding to the C-RNTI of the            terminal or initially receives an indication thereof, the            terminal may determine that the first condition is            satisfied. As another method, if the size of the uplink            transmission resource allocated by the random access            response message is sufficient enough for the terminal to            further transmit uplink data, as well as message 3, the            terminal may determine that an initial uplink transmission            resource has been received, thereby determining that the            first condition is satisfied.    -   In the case where a handover method that does not require a        random access procedure (RACH-less handover) is indicated by the        handover command message received by the terminal    -   if the handover command message includes an uplink transmission        resource to the target base station        -   the terminal may determine that the random access procedure            is successfully completed when the terminal transmits            message 3 (e.g., a handover completion message or an            “RRCReconfigurationComplete” message) using the uplink            transmission resource of the target base station and            receives a UE identity confirmation MAC CE from the base            station, thereby determining that the first condition is            satisfied. As another method, when the terminal receives a            first uplink transmission resource through a PDCCH            corresponding to a C-RNTI of the terminal by monitoring the            PDCCH after a random access procedure is successfully            completed, the terminal may determine that the first            condition is satisfied.    -   In the case where the handover command message does not include        an uplink transmission resource for the target base station        -   when the terminal receives an uplink transmission resource            through a PDCCH corresponding to a C-RNTI of the terminal by            monitoring the PDCCH for the target base station (or cell),            or when the terminal transmits message 3 (e.g., a handover            completion message or an “RRCReconfigurationComplete”            message) using the uplink transmission resource and receives            a UE identity confirmation MAC CE from the base station, the            terminal may determine that the random access procedure is            successfully completed, thereby determining that the first            condition is satisfied. As another method, when the terminal            receives a first uplink transmission resource through a            PDCCH corresponding to a C-RNTI of the terminal by            monitoring the PDCCH after a random access procedure is            successfully completed, the terminal may determine that the            first condition is satisfied.

If the first condition is not satisfied in the above step, the terminalmay repeat to check the first condition while continuing the existingprocedure (1250).

In the disclosure, the operation of the PDCP entity is used to preventthe occurrence of data interruption time and data loss when performingthe embodiments of handover procedures for minimizing the datainterruption time (the first embodiment, the second embodiment, thethird embodiment, the fourth embodiment, the fifth embodiment, or the5-2^(nd) embodiment), which are proposed in the disclosure. Therefore,hereinafter, the disclosure proposes embodiments of a PDCP entityprocedure supporting the handover procedure in order to minimize datainterruption time.

The indicators for the respective embodiments of the handover procedurefor minimizing data interruption time proposed in the disclosure may bedefined. Therefore, the embodiments of the PDCP entity proceduresupporting the handover procedure for minimizing data interruption timeaccording to the disclosure may indicate, to the terminal, the handoverprocedure corresponding to a specific embodiment to be triggered usingthe handover command message (e.g., an RRCReconfiguration message) whenthe base station transmits a handover command message to the terminal(620) in FIG. 6 . Thus, the terminal may perform a handover procedureaccording to the handover method indicated by the handover commandmessage, thereby performing handover to the target base station whileminimizing the data interruption time.

In addition, the indicators for the embodiments of PDCP entityprocedures in the handover procedure for minimizing the datainterruption time proposed in the disclosure may be defined for eachbearer or for each PDCP entity. Therefore, the base station mayindicate, to the terminal, the PDCP entity procedure corresponding to aspecific embodiment to be triggered using a handover command message(e.g., an RRCReconfiguration message), so that the terminal may performthe indicated PDCP entity procedure.

Hereinafter, the disclosure proposes a first embodiment of the PDCPentity procedure supporting a handover procedure for minimizing datainterruption time proposed in the disclosure.

In the first embodiment of the PDCP entity procedure proposed in thedisclosure, the specific operation of a transmitting PDCP entity and areceiving PDCP entity are as follows. The first embodiment may bereferred to as “PDCP re-establishment”.

If an upper layer entity (e.g., an RRC entity) requests the firstembodiment of the PDCP entity procedure for a certain bearer, or if thefirst embodiment of the PDCP entity procedure is indicated by anindicator when the terminal receives a handover command message or an“RRCReconfiguration” message, the transmitting PDCP entity may performat least one of the following procedures.

1. If there is no indicator for UM DRBs and AM DRBs to continue usingthe header compression protocol, the transmitting PDCP entity mayinitialize the header compression protocol and starts the same in aunidirectional (U) mode of an initialized and refreshed (IR) state.

2. The transmitting PDCP entity sets window state variables (e.g.,TX_NEXT) of UM DRBs and SRBs to initial values.

3. As to SRBs, the transmitting PDCP entity discards all of the storeddata (e.g., PDCP SDUs or PDCP PDUs) (since the data is RRC messagesproduced to be transmitted to the source base station, they arediscarded to avoid transmitting the same to the target base station).

4. The transmitting PDCP entity applies a new security key and a newciphering algorithm received from an upper layer entity (e.g., an RRCentity).

5. The transmitting PDCP entity applies a new security key and a newintegrity protection algorithm received from an upper layer entity(e.g., an RRC entity).

6. As to UM DRBs, the transmitting PDCP entity regards the data (e.g.,PDCP SDUs), which has not been transmitted to a lower layer entity eventhough the PDCP serial numbers have already been assigned thereto (afterdiscarding all of the existing stored PDCP PDUs), as the data receivedfrom an upper layer (e.g., an SDAP entity or a TCP/IP entity), andtransmits data in the ascending order of COUNT values (or PDCP serialnumbers) allocated before the re-establishment of the PDCP. In addition,the transmitting PDCP entity does not restart a data discard timer.Specifically, the transmitting PDCP entity performs a new headercompression procedure on the data (PDCP SDUs), performs again anintegrity procedure or a ciphering procedure, configures a PDCP header,and transmits the same to a lower layer entity.

7. As to AM DRBs, the transmitting PDCP entity may perform (afterdiscarding all of the existing stored PDCP PDUs) a new headercompression procedure on the data, in the ascending order of COUNTvalues (or PDCP serial numbers) allocated before the re-establishment ofthe PDCP, from the first data (e.g., PDCP SDUs) of which the successfuldelivery has not been confirmed by lower layer entities (e.g., RLCentities), may perform again an integrity procedure or a cipheringprocedure, configures a PDCP header, and may transmit the same to alower layer entity, thereby performing retransmission or transmission.In other words, the transmitting PDCP entity performs retransmission byaccumulating data from the first data of which the successful deliveryhas not been confirmed.

If an upper layer entity (e.g., an RRC entity) requests the firstembodiment of the PDCP entity procedure for a certain bearer, or if thefirst embodiment of the PDCP entity procedure is indicated when theterminal receives a handover command message or an RRC Reconfigurationmessage, the receiving PDCP entity may perform the following procedures.

1. The receiving PDCP entity processes the data (e.g., PDCP PDUs)received from a lower layer entity due to the re-establishment of thelower layer entities (e.g., RLC entities).

2. As to SRBs, the receiving PDCP entity discards all of the stored data(e.g., PDCP SDUs or PDCP PDUs) (since the data is the RRC messagesreceived from the source base station, the receiving PDCP entitydiscards the date).

3. If a reordering timer is running for SRBs and UM DRBs, the receivingPDCP entity may stop and reset the timer. In addition, the receivingPDCP entity may perform a header decompression procedure on all of thestored data (e.g., PDCP SDUs) for the UM DRBs, and transmits the same toan upper layer entity.

4. If there is no indicator to continue using the header decompressionprotocol for AM DRBs, the receiving PDCP entity performs a headerdecompression procedure on the stored data (e.g., PDCP SDUs).

5. If there is no indicator to continue using the header decompressionprotocol for UM DRBs and AM DRBs, the receiving PDCP entity initializesthe downlink header decompression protocol and starts the same in aunidirectional (U) mode of a no context (NC) state.

6. The receiving PDCP entity sets window variables (e.g., RX_NEXT andRX_DELIV) of UM DRBs and SRBs to initial values.

7. The receiving PDCP entity applies a new security key and a newciphering/deciphering algorithm received from an upper layer entity(e.g., an RRC entity).

8. The receiving PDCP entity applies a new security key and a newintegrity protection/verification algorithm received from an upper layerentity (e.g., an RRC entity).

Hereinafter, the disclosure proposes a second embodiment of the PDCPentity procedure supporting the handover procedure for minimizing datainterruption time proposed in the disclosure.

In the second embodiment of the PDCP entity procedure proposed in thedisclosure, the specific operation of a transmitting PDCP entity and areceiving PDCP entity are as follows. The second embodiment may bereferred to as “PDCP data recovery”.

If an upper layer entity (e.g., an RRC entity) requests the secondembodiment of the PDCP entity procedure for a certain bearer, or if thesecond embodiment of the PDCP entity procedure is indicated by anindicator when the terminal receives a handover command message or an“RRCReconfiguration” message, the transmitting PDCP entity may performat least one of the following procedures.

1. If an upper layer entity (e.g., an RRC entity) requests a PDCP datarecovery procedure for AM DRBs, the transmitting PDCP entity may performthe following procedure.

A. The transmitting PDCP entity may perform selective retransmissiononly on all of the data of which the successful transmission (RLC ACK)has not been confirmed by lower layer entities (e.g., RLC entities),among the data (e.g., PDCP PDUs) previously transmitted to are-established AM RLC entity or a disconnected AM RLC entity, in theascending order of COUNT values (or PDCP serial numbers). Thetransmission and retransmission may be performed on the data previouslystored in a buffer, or in the case of data that has not been generated,the data may be generated, and then transmission and retransmission maybe performed on the generated data.

Hereinafter, the disclosure proposes a third embodiment of the PDCPentity procedure supporting the handover procedure for minimizing datainterruption time proposed in the disclosure.

In the third embodiment of the PDCP entity procedure proposed in thedisclosure, the specific operation of a transmitting PDCP entity and areceiving PDCP entity are as follows. The third embodiment may bereferred to as “PDCP continuation” or another name. In addition, thethird embodiment may be configured as a 3-1^(st) embodiment and a3-2^(nd) embodiment.

If an upper layer entity (e.g., an RRC entity) requests a 3-1^(st)embodiment of a PDCP entity procedure for a certain bearer, or if a3-1^(st) embodiment of a PDCP entity procedure is indicated by anindicator when a terminal receives a handover command message or an“RRCReconfiguration” message, the transmitting PDCP entity may performone or more of a plurality of procedures below.

As another method, if the handover command message contains an indicatorindicating the fourth embodiment or the fifth embodiment proposed in thedisclosure to handover, the transmitting PDCP entity may perform one ormore of a plurality of procedures in the 3-1^(st) embodiment below.

1. If there is no indicator to continue to use the header compressionprotocol for UM DRBs and AM DRBs, the transmitting PDCP entityinitializes the header compression protocol and starts the same in aunidirectional (U) mode of an initialized and refreshed (IR) state.

2. Since the transmission of data to the source base station and thetarget base station continues to be performed in order to minimize datainterruption time, the transmitting PDCP entity does not initializewindow variables (e.g., TX_NEXT) for UM DRBs.

3. If a handover command message is received from the source basestation, since the terminal no longer transmits and receives the RRCmessages to and from the source base station, the terminal may setwindow state variables (e.g., TX_NEXT) of the SRBs to initial values.Alternatively, the terminal may release the SRBs for the source basestation. In addition, the terminal may also set window state variables(e.g., TX_NEXT) to initial values for the SRBs of the second bearer ofthe target base station.

As another method, if the terminal fails to perform handover to thetarget base station, the terminal may transmit a handover failuremessage, an RRC connection re-establishment message, or an RRCconnection recovery request message through the SRB of the source basestation while maintaining a SRB connection with the source base station,thereby reducing the transmission delay due to the failure of handover.As another method, when maintaining the SRB connection with the sourcebase station above, the terminal may discard the data (PDCP SDUs or PDCPPDUs) stored in the SRB, and may set a window state variable thereof(e.g., TX_NEXT) to an initial value, thereby preventing the occurrenceof a gap of PDCP serial numbers when transmitting a handover failuremessage, an RRC connection re-establishment message, or an RRCconnection recovery message through the SRB of the source base stationlater.

4. If a handover command message is received from the source basestation, the terminal may discard all of the stored data (e.g., PDCPSDUs or PDCP PDUs) for SRBs because RRC messages need no longer to betransmitted to and received from the source base station (since the datais RRC messages produced and intended to be transmitted to the sourcebase station, the terminal discards the data in order to avoidtransmitting the same to the target base station).

5. The transmitting PDCP entity may store and prepare a new security keyand a new ciphering algorithm received from an upper layer entity (e.g.,an RRC entity), and may apply the same to a PDCP entity of the secondbearer for the target base station and to data. In addition, thetransmitting PDCP entity may continue to apply the old security key andciphering algorithm, used before receiving the new security key andciphering algorithm from the upper layer entity, to the PDCP entity ofthe first bearer for the source base station and to data.

6. The transmitting PDCP entity may store and prepare a new security keyand a new integrity protection algorithm received from an upper layerentity (e.g., an RRC entity), and may apply the same to the PDCP entityof the second bearer for the target base station and to data. Inaddition, the transmitting PDCP entity may continue to apply thesecurity key and integrity protection algorithm, used before receivingthe new security key and integrity protection algorithm from the upperlayer entity, to the PDCP entity of the first bearer for the source basestation and to data.

7. The transmitting PDCP entity may transmit a PDCP status report to thesource base station, thereby reporting the currenttransmission/reception state of data (e.g., successful or unsuccessfulreception of data), when it is necessary, configured, or indicated, orat all times.

8. If a user data compression procedure (e.g., uplink data compression)is indicated to the terminal, the terminal may initialize a buffer forthe user data compression procedure for the source base station or thetarget base station. As another method, if dictionary information ispredefined, the buffer may be initialized using the dictionaryinformation. As another method, if there is no indicator to continueusing the buffer content for the user data compression procedure, theterminal may initialize the buffer. As another method, if dictionaryinformation is predefined, the buffer may be initialized using thedictionary information. In addition, the terminal may apply user datacompression configuration for the target base station, and mayinitialize the buffer.

If an upper layer entity (for example, an RRC entity) requests a3-1^(st) embodiment of a PDCP entity procedure for a certain bearer, orif a 3-1^(st) embodiment of a PDCP entity procedure is indicated by anindicator when a terminal receives a handover command message or an RRCReconfiguration message, the receiving PDCP entity may perform one ormore of the following procedures.

As another method, if the handover command message contains an indicatorindicating the fourth embodiment or the fifth embodiment proposed in thedisclosure to handover, the receiving PDCP entity may perform one ormore of a plurality of procedures in the 3-1^(st) embodiment below.

1. If a handover command message is received from the source basestation, since the terminal no longer transmits and receives the RRCmessages to and from the source base station, the terminal may discardall of the stored data (e.g., PDCP SDUs or PDCP PDUs) for SRBs (the datais discarded because the data is the RRC messages received from thesource base station).

In addition, the terminal may stop the reordering timer if thereordering timer is running. Alternatively, the terminal may releaseSRBs for the source base station. In addition, the terminal may setwindow variables (e.g., RX_NEXT and RX_DELIV) to initial values for SRBsof the second bearer for the target base station, and may stop thereordering timer if the reordering timer is running.

As another method, if the terminal fails to perform handover to thetarget base station, the terminal may transmit a handover failuremessage, an RRC connection re-establishment message, or an RRCconnection recovery request message through the SRB of the source basestation while maintaining a SRB connection with the source base station,thereby reducing the transmission delay due to the failure of handover.As another method, when maintaining the SRB connection with the sourcebase station above, the terminal may discard the data (PDCP SDUs or PDCPPDUs) stored in the SRB, and may set window state variables (e.g.,RX_NEXT) to initial values, thereby preventing the occurrence of a gapof PDCP serial numbers when transmitting a handover failure message, anRRC connection re-establishment message, or an RRC connection recoverymessage to the SRB of the source base station later.

2. Since data continues to be received from the source base station orthe target base station without interruption of transmission andreception of data, a separate process on the stored data for UM DRBs isnot required.

3. If a reordering timer is running for SRBs and UM DRBs, the receivingPDCP entity stops and resets the timer, performs a header decompressionprocedure on all of the stored data (e.g., PDCP SDUs) for the UM DRBs,and transmits the same to an upper layer entity.

4. If there is no indicator to continue using the header decompressionprotocol for AM DRBs, the receiving PDCP entity performs a headerdecompression procedure on the stored data (e.g., PDCP SDUs).

5. If there is no indicator to continue using the header decompressionprotocol for UM DRBs and AM DRBs, the receiving PDCP entity initializesdownlink header decompression protocol and starts in a unidirectional(U) mode of a no context (NC) state.

6. Since data continues to be received from the source base station orthe target base station without interruption of transmission andreception of data, the receiving PDCP entity does not set window statevariables (e.g., RX_NEXT and RX_DELIV) to initial values for UM DRBs.

7. The receiving PDCP entity may store and prepare a new security keyand a new ciphering algorithm received from an upper layer entity (e.g.,an RRC entity), and may apply the same to the PDCP entity of the secondbearer for the target base station and to data. In addition, thereceiving PDCP entity may continue to apply the security key andciphering algorithm, used before receiving the new security key andciphering algorithm from the upper layer entity, to the PDCP entity ofthe first bearer for the source base station and to data.

8. The receiving PDCP entity may store and prepare a new security keyand a new integrity protection algorithm received from an upper layerentity (e.g., an RRC entity), and may apply the same to the PDCP entityof the second bearer for the target base station and to data. Inaddition, the receiving PDCP entity may continue to apply the securitykey and integrity protection algorithm, used before receiving the newsecurity key and integrity protection algorithm from the upper layerentity, to the PDCP entity of the first bearer for the source basestation and to data.

9. Upon receiving a PDCP status report, the receiving PDCP entity mayread the current state of transmitting and receiving data to and fromthe source base station or the target base station (e.g., successful orunsuccessful reception of data), and may reflect the same to thetransmission or retransmission of data. For example, the receiving PDCPentity may not perform transmission or retransmission on the data ofwhich the successful delivery is confirmed.

10. If a user data compression procedure (e.g., uplink data compression)is indicated to the terminal, the terminal may initialize a buffer forthe user data compression procedure for the source base station or thetarget base station. As another method, if dictionary information ispredefined, the buffer may be initialized using the dictionaryinformation. As another method, if there is no indicator to continueusing the buffer content for the user data compression procedure, theterminal may initialize the buffer. As another method, if dictionaryinformation is predefined, the buffer may be initialized using thedictionary information. In addition, the terminal may apply user datacompression configuration for the target base station, and mayinitialize the buffer.

If an upper layer entity (for example, an RRC entity) requests a3-2^(nd) embodiment of a PDCP entity procedure for a certain bearer, orif a third condition is satisfied, the transmitting PDCP entity of theterminal may perform the 3-2^(nd) embodiment and specifically, performone or more of a plurality of procedures below.

1. The transmitting PDCP entity may apply a new security key and a newciphering algorithm, which are received from an upper layer entity(e.g., an RRC entity) and stored, to a PDCP entity of the second bearerfor the target base station and to data. In addition, the transmittingPDCP entity may release or discard the security key and cipheringalgorithm applied to the PDCP entity of the first bearer for the sourcebase station before receiving the new security key and cipheringalgorithm from the upper layer entity.

As another method, the transmitting PDCP entity may release the securitykey and ciphering algorithm applied to the PDCP entity of the firstbearer for the source base station in the uplink, and may maintain andapply the same to the data received in the downlink. As another method,the transmitting PDCP entity may discard or release the security key andciphering algorithm applied to the PDCP entity of the first bearer forthe source base station when completely releasing the connection withthe source base station (e.g., downlink data reception), and maycontinue to apply the same to the data received from or transmitted tothe source base station until the connection with the source basestation is completely released.

2. The transmitting PDCP entity may apply a new security key andintegrity verification algorithm, which are received from an upper layerentity (e.g., an RRC entity) and stored, to the PDCP entity of thesecond bearer for the target base station and to data. In addition, thetransmitting PDCP entity may release or discard the security key andciphering algorithm applied to the PDCP entity of the first bearer forthe source base station before receiving the new security key andintegrity verification algorithm from the upper layer entity.

As another method, the transmitting PDCP entity may release the securitykey and ciphering algorithm applied to the PDCP entity of the firstbearer for the source base station in the uplink, and may maintain andapply the same to the data received in the downlink. As another method,the transmitting PDCP entity may discard or release the security key andciphering algorithm applied to the PDCP entity of the first bearer forthe source base station when completely releasing the connection withthe source base station (e.g., downlink data reception), and maycontinue to apply the same to the data received from or transmitted tothe source base station until the connection with the source basestation is completely released.

3. The transmitting PDCP entity may transmit a PDCP status report to thesource base station or the target base station, thereby reporting thecurrent transmission/reception state of data (e.g., successful orunsuccessful reception of data), when it is necessary, configured, orindicated, or at all times.

4. As to UM DRBs, the transmitting PDCP entity regards the data (e.g.,PDCP SDUs), which has not been transmitted to a lower layer entity eventhough the PDCP serial numbers have already been assigned thereto (afterdiscarding all of the existing stored PDCP PDUs), as the data receivedfrom an upper layer (e.g., an SDAP entity or a TCP/IP entity), andtransmits data in the ascending order of COUNT values (or PDCP serialnumbers) allocated before the re-establishment of the PDCP. In addition,the terminal does not restart a data discard timer. Specifically, thetransmitting PDCP entity performs a new header compression procedure onthe data (PDCP SDUs), performs again an integrity procedure or aciphering procedure, configures a PDCP header, and transmits the same toa lower layer entity.

5. As to AM DRBs, the transmitting PDCP entity may perform (afterdiscarding all of the existing stored PDCP PDUs) a new headercompression procedure on the data, in the ascending order of COUNTvalues (or PDCP serial numbers) allocated before the re-establishment ofthe PDCP (or before the third condition is satisfied or before the RRCmessage is received), from the first data (e.g., PDCP SDUs) of which thesuccessful delivery has not been confirmed by lower layer entities(e.g., RLC entities). In addition, the transmitting PDCP entity mayperform again an integrity procedure or a ciphering procedure,configures a PDCP header, and may transmit the same to a lower layerentity, thereby performing retransmission or transmission. That is, thetransmitting PDCP entity performs retransmission by accumulating datafrom the first data of which the successful delivery has not beenconfirmed.

As another method, the transmitting PDCP entity may performretransmission only on the data of which the successful delivery has notbeen confirmed by lower layer entities (e.g., RLC entities) whenperforming retransmission. More specifically, as to AM DRBs, thetransmitting PDCP entity (after discarding all of the PDCP PDUs storedto be transmitted to the source base station through a first protocolentity previously connected to the PDCP entity) may release the lowerlayer entities (e.g., an RLC entity or a MAC entity), which are thefirst protocol entities for transmitting data to the source basestation. In addition, the transmitting PDCP entity may performs a newheader or data compression procedure from the first data (e.g., PDCPSDUs) of which the successful transmission has not been confirmed bylower layer entities (e.g., RLC entities), which are the first protocolentities for transmitting data to the source base station, in theascending order of COUNT values (or PDCP serial numbers) allocatedbefore the re-establishment of the PDCP (or before the third conditionis satisfied or before the RRC message is received) by applying thesecurity key or the header compression (or data compression) protocolcontext corresponding to the target base station, may perform theintegrity or ciphering procedure again, may configure a PDCP header, andmay transmit the PDCP header to the lower layer entity, which is thesecond protocol entity for transmitting data to the target base station,thereby performing retransmission or transmission. That is, thetransmitting PDCP entity may perform retransmission by accumulating datafrom the first data of which the successful delivery has not beenconfirmed.

As another method, when performing the retransmission, the transmittingPDCP entity transmits only the data of which the successful delivery hasnot been confirmed by lower layer entities (e.g., RLC entities), whichare the first protocol entities for transmitting data to the source basestation, to the lower layer entity, which is the second protocol entityfor transmitting data to the target base station, thereby performingselective retransmission. As another method, the transmission orretransmission operation may be performed after releasing the lowerlayer entities (e.g., RLC entities or MAC entities), which are firstprotocol entities for transmitting data to the source base station.

6. If a user data compression procedure (e.g., uplink data compression)is indicated to the terminal, the terminal may initialize a buffer forthe user data compression procedure for the source base station or thetarget base station. As another method, if dictionary information ispredefined, the buffer may be initialized using the dictionaryinformation. In addition, the terminal may release user data compressionconfiguration for the source base station, and may release the buffer.

If an upper layer entity (for example, an RRC entity) requests a3-2^(nd) embodiment of a PDCP entity procedure for a certain bearer, orif a third condition is satisfied, the receiving PDCP entity of theterminal may perform the 3-2^(nd) embodiment and specifically, performone or more of a plurality of procedures below.

1. If there is data (e.g., PDCP PDUs) received from lower layer entitiesdue to re-establishment of lower layer entities (e.g., RLC entities),the receiving PDCP entity processes the data.

2. Since data continues to be received from the source base station orthe target base station without interruption of transmission andreception of data, a separate process on the stored data for UM DRBs isnot required.

3. Since data continues to be received from the source base station orthe target base station without interruption of transmission andreception of data, the receiving PDCP entity does not set window statevariables (e.g., RX_NEXT and RX_DELIV) to initial values for AM DRBs orUM DRBs.

4. The receiving PDCP entity may apply a new security key and a newciphering algorithm, which are received from an upper layer entity(e.g., an RRC entity) and stored, to the PDCP entity of the secondbearer for the target base station and to data. In addition, if downlinkdata is no longer received from the source base station, the receivingPDCP entity may release or discard the security key and cipheringalgorithm applied to the PDCP entity of the first bearer for the sourcebase station before receiving the new security key and cipheringalgorithm from the upper layer entity.

As another method, the receiving PDCP entity may release the securitykey and ciphering algorithm applied to the PDCP entity of the firstbearer for the source base station in the uplink, and may maintain andapply the same to the data received in the downlink. As another method,the receiving PDCP entity may discard or release the security key andciphering algorithm applied to the PDCP entity of the first bearer forthe source base station when completely releasing the connection withthe source base station (e.g., downlink data reception), and maycontinue to apply the same to the data received from or transmitted tothe source base station until the connection with the source basestation is completely released.

5. The receiving PDCP entity may apply a new security key and a newintegrity verification algorithm, which are received from an upper layerentity (e.g., an RRC entity) and stored, to the PDCP entity of thesecond bearer for the target base station and to data. In addition, ifdownlink data is no longer received from the source base station, thereceiving PDCP entity may release or discard the security key andciphering algorithm applied to the PDCP entity of the first bearer forthe source base station before receiving the new security key andintegrity verification algorithm from the upper layer entity.

As another method, the receiving PDCP entity may release the securitykey and ciphering algorithm applied to the PDCP entity of the firstbearer for the source base station in the uplink, and may maintain andapply the same to the data received in the downlink. As another method,the receiving PDCP entity may discard or release the security key andciphering algorithm applied to the PDCP entity of the first bearer forthe source base station when completely releasing the connection withthe source base station (e.g., downlink data reception), and maycontinue to apply the same to the data received from or transmitted tothe source base station until the connection with the source basestation is completely released.

6. Upon receiving a PDCP status report, the receiving PDCP entity mayread the current state of transmitting and receiving data to and fromthe source base station or the target base station (e.g., successful orunsuccessful reception of data), and may reflect the same to thetransmission or retransmission of data. For example, the receiving PDCPentity may not perform transmission or retransmission on the data ofwhich the successful delivery is confirmed.

7. If a user data compression procedure (e.g., uplink data compression)is indicated to the terminal, the terminal may initialize a buffer forthe user data compression procedure for the source base station or thetarget base station. As another method, if dictionary information ispredefined, the buffer may be initialized using the dictionaryinformation. In addition, the terminal may release user data compressionconfiguration for the source base station, and may release the buffer.

The third condition may be one of the following conditions.

-   -   When the terminal performs a procedure of random access to the        target base station through the entities of the second bearer        and receives a random access response    -   when the terminal performs a procedure of random access to the        target base station through the entities of the second bearer,        receives a random access response, and configures and transmits        a handover completion message to the target base station    -   when the terminal completes a procedure of random access to the        target base station through the entities of the second bearer        and initially transmits data using a PUCCH or PUSCH uplink        transmission resource    -   when the base station configures a separate timer for the        terminal through an RRC message and the timer expires    -   the timer may start when the terminal receives a handover        command message from the source base station, when the terminal        starts random access to the target base station (when        transmitting a preamble), when the terminal receives a random        access response from the target base station, when the terminal        transmits a handover completion message to the target base        station, or when the terminal initially transmits data using a        PUCCH or PUSCH uplink transmission resource.    -   when the terminal performs a procedure of random access to the        target base station through the entities of the second bearer,        receives a random access response, and configures and transmits        a handover completion message to the target base station and        when the successful transmission of the handover completion        message is identified by a MAC entity (HARQ ACK) or an RLC        entity (RLC ACK)    -   when the terminal performs a procedure of random access to the        target base station through the entities of the second bearer,        receives a random access response, configures and transmits a        handover completion message to the target base station, and        receives an uplink transmission resource that is initially        allocated by the target base station, or receives an initial        indication of an uplink transmission resource    -   when the terminal receives, from the target base station, an        indication {e.g., an RRC message (e.g., an “RRCReconfiguration”        message), a MAC CE, an RLC control PDU, or a PDCP control PDU}        to switch the uplink to the target base station    -   when the terminal receives, from the source base station, an        indication {e.g., an RRC message (e.g., an “RRCReconfiguration”        message), a MAC CE, an RLC control PDU, or a PDCP control PDU}        to switch the uplink to the target base station    -   when the terminal receives, from the target base station, an        indication {e.g., an RRC message (e.g., an “RRCReconfiguration”        message), a MAC CE, an RLC control PDU, or a PDCP control PDU}        to release the connection with the source base station    -   when the terminal receives, from the source base station, an        indication {e.g., an RRC message (e.g., an “RRCReconfiguration”        message), a MAC CE, an RLC control PDU, or a PDCP control PDU}        to release the connection with the source base station    -   when the terminal releases the connection with the source base        station and releases first protocol entities    -   when the terminal fails to receive downlink data from the source        base station for a predetermined period of time    -   when the terminal successfully completes a procedure of random        access to the target base station through the entities of the        second bearer, and receives a first uplink transmission resource        allocated by the target base station, or receives an indication        of a first uplink transmission resource    -   for example, more specifically, if the terminal is instructed to        perform random access to the target base station by receiving a        handover command message from the source base station, and if        the indicated random access is contention-free random access        (CFRA) {for example, if a predetermined preamble or a UE cell        identifier (e.g., C-RNTI) is allocated}        -   the terminal may determine that the random access procedure            is successfully completed when the terminal transmits a            predetermined preamble to the cell of the target base            station and receives a random access response (RAR) message.            Accordingly, when the terminal receives a first uplink            transmission resource allocated, included, or indicated in            the random access response message, the terminal may            determine that the third condition is satisfied.    -   If the terminal is instructed to perform random access to the        target base station by receiving a handover command message from        the source base station, and if the indicated random access is        contention-based random access (CBRA) {for example, if a        predetermined preamble or a UE cell identifier (e.g., C-RNTI) is        not allocated)        -   the terminal may determine that the procedure of random            access to the target base station is successfully completed            when the terminal transmits a preamble (e.g., an arbitrary            preamble) to the cell of the target base station, receives a            random access response (RAR) message, transmits message 3            (e.g., a handover completion message) using the uplink            transmission resource allocated, included, or indicated in            the random access response message, and receives a MAC CE            (contention resolution MAC CE) indicating that the            contention has been resolved from the target base station.            Accordingly, thereafter, when the terminal monitors a PDCCH            and initially receives an uplink transmission resource            through the PDCCH corresponding to the C-RNTI of the            terminal or initially receives an indication thereof, the            terminal may determine that the third condition is            satisfied. As another method, if the size of the uplink            transmission resource allocated by the random access            response message is sufficient enough for the terminal to            further transmit uplink data, as well as message 3, the            terminal may determine that an initial uplink transmission            resource has been received, thereby determining that the            third condition is satisfied.    -   In the case where a handover method that does not require a        random access procedure (RACH-less handover) is indicated by the        handover command message received by the terminal    -   if the handover command message includes an uplink transmission        resource to the target base station        -   the terminal may determine that the random access procedure            is successfully completed when the terminal transmits            message 3 (e.g., a handover completion message or an            “RRCReconfigurationComplete” message) using the uplink            transmission resource of the target base station and            receives a UE identity confirmation MAC CE from the base            station, thereby determining that the third condition is            satisfied. As another method, when the terminal receives a            first uplink transmission resource through a PDCCH            corresponding to a C-RNTI of the terminal by monitoring the            PDCCH after a random access procedure is successfully            completed, the terminal may determine that the third            condition is satisfied.    -   In the case where the handover command message does not include        an uplink transmission resource for the target base station        -   when the terminal receives an uplink transmission resource            through a PDCCH corresponding to a C-RNTI of the terminal by            monitoring the PDCCH for the target base station (or cell),            or when the terminal transmits message 3 (e.g., a handover            completion message or an “RRCReconfigurationComplete”            message) using the uplink transmission resource and receives            a UE identity confirmation MAC CE from the base station, the            terminal may determine that the random access procedure is            successfully completed, thereby determining that the third            condition is satisfied. As another method, when the terminal            receives a first uplink transmission resource through a            PDCCH corresponding to a C-RNTI of the terminal by            monitoring the PDCCH after a random access procedure is            successfully completed, the terminal may determine that the            third condition is satisfied.

If the third condition is not satisfied in the above step, the terminalmay repeat to check the third condition while continuing the existingprocedure.

Hereinafter, the disclosure proposes a fourth embodiment of the PDCPentity procedure supporting the handover procedure for minimizing datainterruption time proposed in the disclosure.

In the fourth embodiment of the PDCP entity procedure proposed in thedisclosure, the specific operation of a transmitting PDCP entity and areceiving PDCP entity are as follows. The fourth embodiment may bereferred to as “PDCP continuation” or another name. In addition, thefourth embodiment may be configured as a 4-1^(st) embodiment and a4-2^(nd) embodiment. In the fourth embodiment, since data continues tobe transmitted and received, a procedure for initializing or handling aheader compression protocol is unnecessary.

If an upper layer entity (e.g., an RRC entity) requests a 4-1^(st)embodiment of a PDCP entity procedure for a certain bearer, or if a4-1^(st) embodiment of a PDCP entity procedure is indicated by anindicator when a terminal receives a handover command message or an“RRCReconfiguration” message, the transmitting PDCP entity may performone or more of a plurality of procedures below.

As another method, if the handover command message contains an indicatorto apply the fourth embodiment or the fifth embodiment proposed in thedisclosure to handover, the transmitting PDCP entity may perform one ormore of a plurality of procedures in the 4-1^(st) embodiment below.

1. Since the transmission of data to the source base station and thetarget base station continues to be performed in order to minimize datainterruption time, the transmitting PDCP entity does not initializewindow variables (e.g., TX_NEXT) for UM DRBs.

2. If a handover command message is received from the source basestation, since the terminal no longer transmits and receives the RRCmessages to and from the source base station, the terminal may setwindow state variables (e.g., TX_NEXT) to initial values for SRBs, ormay release SRBs for the source base station. In addition, the terminalmay set window state variables (e.g., TX_NEXT) to initial values forSRBs of the second bearer of the target base station.

3. If a handover command message is received from the source basestation, the terminal may discard all of the stored data (e.g., PDCPSDUs or PDCP PDUs) for SRBs because RRC messages need no longer to betransmitted to and received from the source base station (since the datais RRC messages produced and intended to be transmitted to the sourcebase station, the terminal discards the data in order to avoidtransmitting the same to the target base station).

As another method, if the terminal fails to perform handover to thetarget base station, the transmitting PDCP entity may transmit ahandover failure message, an RRC connection re-establishment message, oran RRC connection recovery request message through the SRB of the sourcebase station while maintaining a SRB connection with the source basestation, thereby reducing the transmission delay due to the failure ofhandover. As another method, when maintaining the SRB connection withthe source base station above, the terminal may discard the data (PDCPSDUs or PDCP PDUs) stored in the SRB, and may set a window statevariable (e.g., TX_NEXT) to an initial value, thereby preventing theoccurrence of a gap of PDCP serial numbers when transmitting a handoverfailure message, an RRC connection re-establishment message, or an RRCconnection recovery message to the SRB of the source base station later.

4. The transmitting PDCP entity may store and prepare a new security keyand a new ciphering algorithm received from an upper layer entity (e.g.,an RRC entity), and may apply the same to a PDCP entity of the secondbearer for the target base station and to data. In addition, thetransmitting PDCP entity may continue to apply the security key andciphering algorithm, used before receiving the new security key andciphering algorithm from the upper layer entity, to the PDCP entity ofthe first bearer for the source base station and to data.

5. The transmitting PDCP entity may store and prepare a new security keyand a new integrity protection algorithm received from an upper layerentity (e.g., an RRC entity), and may apply the same to the PDCP entityof the second bearer for the target base station and to data. Inaddition, the transmitting PDCP entity may continue to apply thesecurity key and integrity protection algorithm, used before receivingthe new security key and integrity protection algorithm from the upperlayer entity, to the PDCP entity of the first bearer for the source basestation and to data.

6. The transmitting PDCP entity may transmit a PDCP status report to thesource base station, thereby reporting the currenttransmission/reception state of data (e.g., successful or unsuccessfulreception of data), when it is necessary, configured, or indicated, orat all times.

7. If a user data compression procedure (e.g., uplink data compression)is indicated to the terminal, the terminal may initialize a buffer forthe user data compression procedure for the source base station or thetarget base station. As another method, if dictionary information ispredefined, the buffer may be initialized using the dictionaryinformation. As another method, if there is no indicator to continueusing the buffer content for the user data compression procedure, theterminal may initialize the buffer. As another method, if dictionaryinformation is predefined, the buffer may be initialized using thedictionary information. In addition, the terminal may apply user datacompression configuration for the target base station, and mayinitialize the buffer.

If an upper layer entity (for example, an RRC entity) requests a4-1^(st) embodiment of a PDCP entity procedure for a certain bearer, orif a 4-1^(st) embodiment of a PDCP entity procedure is indicated by anindicator when a terminal receives a handover command message or an RRCReconfiguration message, the receiving PDCP entity may perform one ormore of a plurality of procedures below.

As another method, if the handover command message contains an indicatorindicating the fourth embodiment or the fifth embodiment proposed in thedisclosure to handover, the receiving PDCP entity may perform one ormore of a plurality of procedures in the 4-1^(st) embodiment below.

1. If a handover command message is received from the source basestation, since the terminal no longer transmits and receives the RRCmessages to and from the source base station, the terminal may discardall of the stored data (e.g., PDCP SDUs or PDCP PDUs) for SRBs (the datais discarded because the data is the RRC messages received from thesource base station).

In addition, the terminal may stop the reordering timer if thereordering timer is running. Alternatively, the terminal may releaseSRBs for the source base station. In addition, the terminal may setwindow variables (e.g., RX_NEXT and RX_DELIV) to initial values for SRBsof the second bearer for the target base station, and may stop thereordering timer if the reordering timer is running.

As another method, if the terminal fails to perform handover to thetarget base station, the terminal may transmit a handover failuremessage, an RRC connection re-establishment message, or an RRCconnection recovery request message to the SRB of the source basestation while maintaining a SRB connection with the source base station,thereby reducing the transmission delay due to the failure of handover.As another method, when maintaining the SRB connection with the sourcebase station above, the receiving PDCP entity may discard the data (PDCPSDUs or PDCP PDUs) stored in the SRB, and may set a window statevariable (e.g., RX_NEXT) to an initial value, thereby preventing theoccurrence of a gap of PDCP serial numbers when transmitting a handoverfailure message, an RRC connection re-establishment message, or an RRCconnection recovery message to the SRB of the source base station later.

2. Since data continues to be received from the source base station orthe target base station without interruption of transmission andreception of data, a separate process on the stored data for UM DRBs isnot required.

3. Since data continues to be received from the source base station orthe target base station without interruption of transmission andreception of data, the receiving PDCP entity does not set window statevariables (e.g., RX_NEXT and RX_DELIV) to initial values for UM DRBs.

4. The receiving PDCP entity may store and prepare a new security keyand a new ciphering algorithm received from an upper layer entity (e.g.,an RRC entity), and may apply the same to the PDCP entity of the secondbearer for the target base station and to data. In addition, thereceiving PDCP entity may continue to apply the security key andciphering algorithm, used before receiving the new security key andciphering algorithm from the upper layer entity, to the PDCP entity ofthe first bearer for the source base station and to data.

5. The receiving PDCP entity may store and prepare a new security keyand a new integrity protection algorithm received from an upper layerentity (e.g., an RRC entity), and may apply the same to the PDCP entityof the second bearer for the target base station and to data. Inaddition, the receiving PDCP entity may continue to apply the securitykey and integrity protection algorithm, used before receiving the newsecurity key and integrity protection algorithm from the upper layerentity, to the PDCP entity of the first bearer for the source basestation and to data.

6. Upon receiving a PDCP status report, the receiving PDCP entity mayread the current state of transmitting and receiving data to and fromthe source base station or the target base station (e.g., successful orunsuccessful reception of data), and may reflect the same to thetransmission or retransmission of data. For example, the receiving PDCPentity may not perform transmission or retransmission on the data ofwhich the successful delivery is confirmed.

7. If a user data compression procedure (e.g., uplink data compression)is indicated to the terminal, the terminal may initialize a buffer forthe user data compression procedure for the source base station or thetarget base station. As another method, if dictionary information ispredefined, the buffer may be initialized using the dictionaryinformation. As another method, if there is no indicator to continueusing the buffer content for the user data compression procedure, theterminal may initialize the buffer. As another method, if dictionaryinformation is predefined, the buffer may be initialized using thedictionary information. In addition, the terminal may apply user datacompression configuration for the target base station, and mayinitialize the buffer.

If an upper layer entity (for example, an RRC entity) requests a4-2^(nd) embodiment of a PDCP entity procedure for a certain bearer, orif a third condition is satisfied, the transmitting PDCP entity of theterminal may perform the 4-2^(nd) embodiment and specifically, performone or more of a plurality of procedures below.

1. The transmitting PDCP entity may apply a new security key and a newciphering algorithm, which are received from an upper layer entity(e.g., an RRC entity) and stored, to a PDCP entity of the second bearerfor the target base station and to data. In addition, the transmittingPDCP entity may release or discard the security key and cipheringalgorithm applied to the PDCP entity of the first bearer for the sourcebase station before receiving the new security key and cipheringalgorithm from the upper layer entity.

As another method, the transmitting PDCP entity may release the securitykey and ciphering algorithm applied to the PDCP entity of the firstbearer for the source base station in the uplink, and may maintain andapply the same to the data received in the downlink. As another method,the transmitting PDCP entity may discard or release the security key andciphering algorithm applied to the PDCP entity of the first bearer forthe source base station when completely releasing the connection withthe source base station (e.g., downlink data reception), and maycontinue to apply the same to the data received from or transmitted tothe source base station until the connection with the source basestation is completely released.

2. The transmitting PDCP entity may apply a new security key and a newintegrity verification algorithm, which are received from an upper layerentity (e.g., an RRC entity) and stored, to the PDCP entity of thesecond bearer for the target base station and to data. In addition, thetransmitting PDCP entity may release or discard the security key andciphering algorithm applied to the PDCP entity of the first bearer forthe source base station before receiving the new security key andintegrity verification algorithm from the upper layer entity.

As another method, the transmitting PDCP entity may release the securitykey and ciphering algorithm applied to the PDCP entity of the firstbearer for the source base station in the uplink, and may maintain andapply the same to the data received in the downlink. As another method,the transmitting PDCP entity may discard or release the security key andciphering algorithm applied to the PDCP entity of the first bearer forthe source base station when completely releasing the connection withthe source base station (e.g., downlink data reception), and maycontinue to apply the same to the data received from or transmitted tothe source base station until the connection with the source basestation is completely released.

3. The transmitting PDCP entity may transmit a PDCP status report to thesource base station or the target base station, thereby reporting thecurrent transmission/reception state of data (e.g., successful orunsuccessful reception of data), when it is necessary, configured, orindicated, or at all times.

4. As to UM DRBs, the transmitting PDCP entity regards the data (e.g.,PDCP SDUs), which has not been transmitted to a lower layer entity eventhough the PDCP serial numbers have already been assigned thereto (afterdiscarding all of the existing stored PDCP PDUs), as the data receivedfrom an upper layer (e.g., an SDAP entity or a TCP/IP entity), andtransmits data in the ascending order of COUNT values (or PDCP serialnumbers) allocated before the re-establishment of the PDCP. In addition,the transmitting PDCP entity does not restart a data discard timer.Specifically, the transmitting PDCP entity performs a new headercompression procedure on the data (PDCP SDUs), performs again anintegrity procedure or a ciphering procedure, configures a PDCP header,and transmits the same to a lower layer entity.

5. As to AM DRBs, the transmitting PDCP entity may perform (afterdiscarding all of the existing stored PDCP PDUs) a new headercompression procedure on the data, in the ascending order of COUNTvalues (or PDCP serial numbers) allocated before the re-establishment ofthe PDCP (or before the third condition is satisfied or before the RRCmessage is received), from the first data (e.g., PDCP SDUs) of which thesuccessful delivery has not been confirmed by lower layer entities(e.g., RLC entities). In addition, the transmitting PDCP entity mayperform again an integrity procedure or a ciphering procedure,configures a PDCP header, and may transmit the same to a lower layerentity, thereby performing retransmission or transmission. That is, thetransmitting PDCP entity may perform retransmission by accumulating datafrom the first data of which the successful delivery has not beenconfirmed.

As another method, the transmitting PDCP entity may performretransmission only on the data of which the successful delivery has notbeen confirmed by lower layer entities (e.g., RLC entities) whenperforming retransmission. More specifically, as to AM DRBs, thetransmitting PDCP entity (after discarding all of the PDCP PDUs storedto be transmitted to the source base station through a first protocolentity previously connected to the PDCP entity) may release the lowerlayer entities (e.g., an RLC entity or a MAC entity), which are thefirst protocol entities for transmitting data to the source basestation. The transmitting PDCP entity may perform a new header or datacompression procedure from the first data (e.g., PDCP SDUs) of which thesuccessful transmission has not been confirmed by lower layer entities(e.g., RLC entities), which are the first protocol entities fortransmitting data to the source base station, in the ascending order ofCOUNT values (or PDCP serial numbers) allocated before there-establishment of the PDCP (or before the third condition is satisfiedor before the RRC message is received) by applying the security key orthe header compression (or data compression) protocol contextcorresponding to the target base station, may perform the integrity orciphering procedure again, may configure a PDCP header, and may transmitthe PDCP header to the lower layer entity, which is the second protocolentity for transmitting data to the target base station, therebyperforming retransmission or transmission. That is, the transmittingPDCP entity may perform retransmission by accumulating data from thefirst data of which the successful delivery has not been confirmed.

As another method, when performing the retransmission, the transmittingPDCP entity transmits only the data of which the successful delivery hasnot been confirmed by lower layer entities (e.g., RLC entities), whichare the first protocol entities for transmitting data to the source basestation, to the lower layer entity, which is the second protocol entityfor transmitting data to the target base station, thereby performingselective retransmission. As another method, the transmission orretransmission operation may be performed after releasing the lowerlayer entities (e.g., RLC entities), which are the first protocolentities for transmitting data to the source base station.

6. If a user data compression procedure (e.g., uplink data compression)is indicated to the terminal, the terminal may initialize a buffer forthe user data compression procedure for the source base station or thetarget base station. As another method, if dictionary information ispredefined, the buffer may be initialized using the dictionaryinformation. In addition, the terminal may release user data compressionconfiguration for the source base station, and may release the buffer.

If an upper layer entity (for example, an RRC entity) requests a4-2^(nd) embodiment of a PDCP entity procedure for a certain bearer, orif a third condition is satisfied, the receiving PDCP entity of theterminal may perform the 4-2^(nd) embodiment and specifically, performone or more of a plurality of procedures below.

1. If there is data (e.g., PDCP PDUs) received from lower layer entitiesdue to re-establishment of lower layer entities (e.g., RLC entities),the receiving PDCP entity processes the data.

2. Since data continues to be received from the source base station orthe target base station without interruption of transmission andreception of data, a separate process on the stored data for UM DRBs isnot required.

3. Since data continues to be received from the source base station orthe target base station without interruption of transmission andreception of data, the receiving PDCP entity does not set window statevariables (e.g., RX_NEXT and RX_DELIV) to initial values for AM DRBs orUM DRBs.

4. The receiving PDCP entity may apply a new security key and a newciphering algorithm, which are received from an upper layer entity(e.g., an RRC entity) and stored, to the PDCP entity of the secondbearer for the target base station and to data. In addition, if downlinkdata is no longer received from the source base station, the receivingPDCP entity may release or discard the security key and cipheringalgorithm applied to the PDCP entity of the first bearer for the sourcebase station before receiving the new security key and cipheringalgorithm from the upper layer entity. As another method, the receivingPDCP entity may release the security key and ciphering algorithm appliedto the PDCP entity of the first bearer for the source base station inthe uplink, and may maintain and apply the same to the data received inthe downlink.

As another method, the receiving PDCP entity may discard or release thesecurity key and ciphering algorithm applied to the PDCP entity of thefirst bearer for the source base station when completely releasing theconnection with the source base station (e.g., downlink data reception),and may continue to apply the same to the data received from ortransmitted to the source base station until the connection with thesource base station is completely released.

5. The receiving PDCP entity may apply a new security key and a newintegrity verification algorithm, which are received from an upper layerentity (e.g., an RRC entity) and stored, to the PDCP entity of thesecond bearer for the target base station and to data. In addition, ifdownlink data is no longer received from the source base station, thereceiving PDCP entity may release or discard the security key andciphering algorithm applied to the PDCP entity of the first bearer forthe source base station before receiving the new security key andintegrity verification algorithm from the upper layer entity.

As another method, the receiving PDCP entity may release the securitykey and ciphering algorithm applied to the PDCP entity of the firstbearer for the source base station in the uplink, and may maintain andapply the same to the data received in the downlink. As another method,the receiving PDCP entity may discard or release the security key andciphering algorithm applied to the PDCP entity of the first bearer forthe source base station when completely releasing the connection withthe source base station (e.g., downlink data reception), and maycontinue to apply the same to the data received from or transmitted tothe source base station until the connection with the source basestation is completely released.

6. Upon receiving a PDCP status report, the receiving PDCP entity mayread the current state of transmitting and receiving data to and fromthe source base station or the target base station (e.g., successful orunsuccessful reception of data), and may reflect the same to thetransmission or retransmission of data. For example, the receiving PDCPentity may not perform transmission or retransmission on the data ofwhich the successful delivery is confirmed.

7. If a user data compression procedure (e.g., uplink data compression)is indicated to the terminal, the terminal may initialize a buffer forthe user data compression procedure for the source base station or thetarget base station. As another method, if dictionary information ispredefined, the buffer may be initialized using the dictionaryinformation. In addition, the terminal may release user data compressionconfiguration for the source base station, and may release the buffer.

The third condition may be one of the following conditions.

-   -   When the terminal performs a procedure of random access to the        target base station through the entities of the second bearer        and receives a random access response    -   when the terminal performs a procedure of random access to the        target base station through the entities of the second bearer,        receives a random access response, and configures and transmits        a handover completion message to the target base station    -   when the terminal completes a procedure of random access to the        target base station through the entities of the second bearer        and initially transmits data using a PUCCH or PUSCH uplink        transmission resource    -   when the base station configures a separate timer for the        terminal through an RRC message and the timer expires    -   the timer may start when the terminal receives a handover        command message from the source base station, when the terminal        starts random access to the target base station (when        transmitting a preamble), when the terminal receives a random        access response from the target base station, when the terminal        transmits a handover completion message to the target base        station, or when the terminal initially transmits data using a        PUCCH or PUSCH uplink transmission resource.    -   when the terminal performs a procedure of random access to the        target base station through the entities of the second bearer,        receives a random access response, and configures and transmits        a handover completion message to the target base station and        when the successful transmission of the handover completion        message is identified by a MAC entity (HARQ ACK) or an RLC        entity (RLC ACK)    -   when the terminal performs a procedure of random access to the        target base station through the entities of the second bearer,        receives a random access response, configures and transmits a        handover completion message to the target base station, and        receives an uplink transmission resource that is initially        allocated by the target base station, or receives an initial        indication of an uplink transmission resource    -   when the terminal receives, from the target base station, an        indication {e.g., an RRC message (e.g., an “RRCReconfiguration”        message), a MAC CE, an RLC control PDU, or a PDCP control PDU}        to switch the uplink to the target base station    -   when the terminal receives, from the source base station, an        indication {e.g., an RRC message (e.g., an “RRCReconfiguration”        message), a MAC CE, an RLC control PDU, or a PDCP control PDU}        to switch the uplink to the target base station    -   when the terminal receives, from the target base station, an        indication {e.g., an RRC message (e.g., an “RRCReconfiguration”        message), a MAC CE, an RLC control PDU, or a PDCP control PDU}        to release the connection with the source base station    -   when the terminal receives, from the source base station, an        indication {e.g., an RRC message (e.g., an “RRCReconfiguration”        message), a MAC CE, an RLC control PDU, or a PDCP control PDU}        to release the connection with the source base station    -   when the terminal releases the connection with the source base        station and releases first protocol entities    -   when the terminal fails to receive downlink data from the source        base station for a predetermined period of time    -   when the terminal successfully completes a procedure of random        access to the target base station through the entities of the        second bearer, and receives a first uplink transmission resource        allocated by the target base station, or receives an indication        of a first uplink transmission resource    -   for example, more specifically, if the terminal is instructed to        perform random access to the target base station by receiving a        handover command message from the source base station, and if        the indicated random access is contention-free random access        (CFRA) {for example, if a predetermined preamble or a UE cell        identifier (e.g., C-RNTI) is allocated}        -   the terminal may determine that the random access procedure            is successfully completed when the terminal transmits a            predetermined preamble to the cell of the target base            station and receives a random access response (RAR) message.            Accordingly, when the terminal receives a first uplink            transmission resource allocated, included, or indicated in            the random access response message, the terminal may            determine that the third condition is satisfied.    -   If the terminal is instructed to perform random access to the        target base station by receiving a handover command message from        the source base station, and if the indicated random access is        contention-based random access (CBRA) {for example, if a        predetermined preamble or a UE cell identifier (e.g., C-RNTI) is        not allocated)        -   the terminal may determine that the procedure of random            access to the target base station is successfully completed            when the terminal transmits a preamble (e.g., an arbitrary            preamble) to the cell of the target base station, receives a            random access response (RAR) message, transmits message 3            (e.g., a handover completion message) using the uplink            transmission resource allocated, included, or indicated in            the random access response message, and receives a MAC CE            (contention resolution MAC CE) indicating that the            contention has been resolved from the target base station.            Accordingly, thereafter, when the terminal monitors a PDCCH            and initially receives an uplink transmission resource            through the PDCCH corresponding to the C-RNTI of the            terminal or initially receives an indication thereof, the            terminal may determine that the third condition is            satisfied. As another method, if the size of the uplink            transmission resource allocated by the random access            response message is sufficient enough for the terminal to            further transmit uplink data, as well as message 3, the            terminal may determine that an initial uplink transmission            resource has been received, thereby determining that the            third condition is satisfied.    -   In the case where a handover method that does not require a        random access procedure (RACH-less handover) is indicated by the        handover command message received by the terminal    -   if the handover command message includes an uplink transmission        resource to the target base station        -   the terminal may determine that the random access procedure            is successfully completed when the terminal transmits            message 3 (e.g., a handover completion message or an            “RRCReconfigurationComplete” message) using the uplink            transmission resource of the target base station and            receives a UE identity confirmation MAC CE from the base            station, thereby determining that the third condition is            satisfied. As another method, when the terminal receives a            first uplink transmission resource through a PDCCH            corresponding to a C-RNTI of the terminal by monitoring the            PDCCH after a random access procedure is successfully            completed, the terminal may determine that the third            condition is satisfied.    -   In the case where the handover command message does not include        an uplink transmission resource for the target base station        -   when the terminal receives an uplink transmission resource            through a PDCCH corresponding to a C-RNTI of the terminal by            monitoring the PDCCH for the target base station (or cell),            or when the terminal transmits message 3 (e.g., a handover            completion message or an “RRCReconfigurationComplete”            message) using the uplink transmission resource and receives            a UE identity confirmation MAC CE from the base station, the            terminal may determine that the random access procedure is            successfully completed, thereby determining that the third            condition is satisfied. As another method, when the terminal            receives a first uplink transmission resource through a            PDCCH corresponding to a C-RNTI of the terminal by            monitoring the PDCCH after a random access procedure is            successfully completed, the terminal may determine that the            third condition is satisfied.

If the third condition is not satisfied in the above step, the terminalmay repeat to check the third condition while continuing the existingprocedure.

Hereinafter, the disclosure proposes a fifth embodiment of the PDCPentity procedure supporting the handover procedure for minimizing datainterruption time proposed in the disclosure.

In the fifth embodiment of the PDCP entity procedure proposed in thedisclosure, the specific operation of a transmitting PDCP entity and areceiving PDCP entity are as follows. The fifth embodiment may bereferred to as “PDCP continuation” or another name. In addition, thefifth embodiment may be configured as a 5-1^(st) embodiment and a5-2^(nd) embodiment.

In the fifth embodiment of the PDCP entity procedure of the disclosure,in the PDCP entity structure shown in FIG. 11 , respective lower PDCPentities 1121 and 1122 may perform header compression procedures on thedata to be transmitted based on different header compression protocolcontexts (e.g., ROHC contexts), and may ciphering procedures usingdifferent security keys.

Specifically, a first lower PDCP entity 1121 may apply a headercompression procedure to the data (e.g., PDCP SDUs) received from anupper layer, based on a first header compression protocol context (e.g.,ROHC contexts), and may encrypt the data using a first security key (asecurity key configured from a source base station or a target basestation). In addition, a second lower PDCP entity 1122 may apply aheader compression procedure to the data (e.g., PDCP SDUs) received froman upper layer, based on a second header compression protocol context(e.g., ROHC contexts), and may encrypt the data using a second securitykey (the security key configured from the source base station or thetarget base station). The two lower PDCP entities may perform the headercompression procedure and the ciphering procedure in parallel, therebyreducing the data processing time and data interruption time that mayoccur during handover.

In the fifth embodiment of the PDCP entity procedure of the disclosure,referring to the PDCP entity structure shown in FIG. 11 , lower PDCPentities 1121 and 1122 may perform deciphering procedures on the data tobe received using different security keys, and may perform headerdecompression procedures using different header compression protocolcontexts (for example, ROHC contexts).

Specifically, a first lower PDCP entity 1121 may apply a headerdecompression procedure to the data (e.g., PDCP SDUs) received from thelower layer entity, based on a first header compression protocol context(e.g., ROHC contexts), and may decrypt the data using a first securitykey (the security key configured from the source base station or thetarget base station). In addition, a second lower PDCP entity 1122 mayapply a header decompression procedure to the data (e.g., PDCP SDUs)received from the lower layer entity, based on a second headercompression protocol context (e.g., ROHC contexts), and may decrypt thedata using a second security key (the security key configured from thesource base station or the target base station). The two lower PDCPentities may perform the header decompression procedure and thedeciphering procedure in parallel, thereby reducing the data processingtime and data interruption time that may occur during handover.

In the PDCP entity structure described above, if an upper layer entity(e.g., an RRC entity) requests a 5-1^(st) embodiment of a PDCP entityprocedure for a certain bearer, or if a 5-1^(st) embodiment of a PDCPentity procedure is indicated by an indicator when a terminal receives ahandover command message or an “RRCReconfiguration” message, thetransmitting PDCP entity may perform one or more of a plurality ofprocedures below.

As another method, if the handover command message contains an indicatorindicating the fourth embodiment or the fifth embodiment proposed in thedisclosure to handover, the transmitting PDCP entity may perform one ormore of a plurality of procedures in the 3-1^(st) embodiment below.

1. Since the transmission of data to the source base station and thetarget base station continues to be performed in order to minimize datainterruption time, the transmitting PDCP entity does not initializewindow variables (e.g., TX_NEXT) for UM DRBs.

2. If a handover command message is received from the source basestation, since the terminal no longer transmits and receives the RRCmessages to and from the source base station, the terminal may setwindow state variables (e.g., TX_NEXT) to initial values for SRBs, ormay release SRBs for the source base station. In addition, the terminalmay also set window state variables (e.g., TX_NEXT) to initial valuesfor SRBs of the second bearer of the target base station.

3. If a handover command message is received from the source basestation, the terminal may discard all of the stored data (e.g., PDCPSDUs or PDCP PDUs) for SRBs because RRC messages need no longer to betransmitted to and received from the source base station (since the datais RRC messages produced and intended to be transmitted to the sourcebase station, the terminal discards the data in order to avoidtransmitting the same to the target base station).

As another method, if the terminal fails to perform handover to thetarget base station, the terminal may transmit a handover failuremessage, an RRC connection re-establishment message, or an RRCconnection recovery request message to the SRB of the source basestation while maintaining a SRB connection with the source base station,thereby reducing the transmission delay due to the failure of handover.As another method, when maintaining the SRB connection with the sourcebase station above, the terminal may discard the data (PDCP SDUs or PDCPPDUs) stored in the SRB, and may set a window state variable (e.g.,TX_NEXT) to an initial value, thereby preventing the occurrence of a gapof PDCP serial numbers when transmitting a handover failure message, anRRC connection re-establishment message, or an RRC connection recoverymessage to the SRB of the source base station later.

4. The transmitting PDCP entity may store and prepare a new security keyand a new ciphering algorithm received from an upper layer entity (e.g.,an RRC entity), and may apply the same to a PDCP entity of the secondbearer (e.g., a second lower PDCP entity) for the target base stationand to data thereafter. In addition, the transmitting PDCP entity maycontinue to apply the security key and ciphering algorithm, used beforereceiving the new security key and ciphering algorithm from the upperlayer entity, to the PDCP entity of the first bearer (e, a first lowerODCO entity) for the source base station and then to data.

5. The transmitting PDCP entity may store and prepare a new security keyand a new integrity protection algorithm received from an upper layerentity (e.g., an RRC entity), and may apply the same to the PDCP entityof the second bearer (e.g., a second lower PDCP entity) for the targetbase station and to data thereafter. In addition, the transmitting PDCPentity may continue to apply the security key and integrity protectionalgorithm, used before receiving the new security key and integrityprotection algorithm from the upper layer entity, to the PDCP entity ofthe first bearer (e.g., a first lower PDCP entity) for the source basestation and to data.

6. The transmitting PDCP entity may transmit a PDCP status report to thesource base station, thereby reporting the currenttransmission/reception state of data (e.g., successful or unsuccessfulreception of data), when it is necessary, configured, or indicated, orat all times.

7. If a user data compression procedure (e.g., uplink data compression)is indicated to the terminal, the terminal may initialize a buffer forthe user data compression procedure for the source base station or thetarget base station. As another method, if dictionary information ispredefined, the buffer may be initialized using the dictionaryinformation. As another method, if there is no indicator to continueusing the buffer content for the user data compression procedure, theterminal may initialize the buffer. As another method, if dictionaryinformation is predefined, the buffer may be initialized using thedictionary information. In addition, the terminal may apply user datacompression configuration for the target base station, and mayinitialize the buffer.

If an upper layer entity (for example, an RRC entity) requests a5-1^(st) embodiment of a PDCP entity procedure for a certain bearer, orif a 5-1^(st) embodiment of a PDCP entity procedure is indicated by anindicator when a terminal receives a handover command message or an“RRCReconfiguration” message, the receiving PDCP entity may perform oneor more of the following procedures.

As another method, if the handover command message contains an indicatorindicating the fourth embodiment or the fifth embodiment proposed in thedisclosure to handover, the receiving PDCP entity may perform one ormore of a plurality of procedures in the 3-1^(st) embodiment below.

1. If the handover command message has no indicator to continue to usethe header compression protocol for each bearer of UM DRBs and AM DRBs,the terminal may initialize the header compression protocol in a PDCPentity of the second bearer (e.g., a second lower PDCP entity), and maystart the same in a unidirectional (U) mode of an initialized andrefreshed (IR) state.

If the handover command message has an indicator to continue to use theheader compression protocol, the terminal may continue to use the headercompression protocol using the header compression protocol of a PDCPentity of the first bearer (e.g., a first lower PDCP entity) for thesource base station, or may perform a header decompression procedure. Inaddition, the terminal may continue to use the header compressionprotocol context without initializing the header compression protocol inthe PDCP entity of the first bearer (e.g., a first lower PDCP entity)for the source base station.

2. If a handover command message is received from the source basestation, since the terminal no longer transmits and receives the RRCmessages to and from the source base station, the receiving PDCP entitydiscards all of the stored data (e.g., PDCP SDUs or PDCP PDUs) for SRBs(the data is discarded because the data is the RRC message received fromthe source base station). In addition, if a reordering timer is running,the terminal may stop the timer. Alternatively, the terminal may releaseSRBs for the source base station. In addition, the terminal may setwindow variables (e.g., RX_NEXT and RX_DELIV) to initial values for SRBsof the second bearer for the target base station, and may stop thereordering timer if the reordering timer is running.

As another method, if the terminal fails to perform handover to thetarget base station, the terminal may transmit a handover failuremessage, an RRC connection re-establishment message, or an RRCconnection recovery request message to the SRB of the source basestation while maintaining a SRB connection with the source base station,thereby reducing the transmission delay due to the failure of handover.As another method, when maintaining the SRB connection with the sourcebase station above, the receiving PDCP entity may discard the data (PDCPSDUs or PDCP PDUs) stored in the SRB, and may set a window statevariable (e.g., RX_NEXT) to an initial value, thereby preventing theoccurrence of a gap of PDCP serial numbers when transmitting a handoverfailure message, an RRC connection re-establishment message, or an RRCconnection recovery message to the SRB of the source base station later.

3. Since data continues to be received from the source base station orthe target base station without interruption of transmission andreception of data, a separate process on the stored data for UM DRBs isnot required.

4. If a reordering timer is running for SRBs, the terminal may stop andreset the timer. As to AM DRBs, if there is no indicator to continueusing the header decompression protocol for the PDCP entity {e.g., asecond (or first) lower PDCP entity} corresponding to a second (orfirst) bearer for the target (or source) base station, the receivingPDCP entity may perform a header decompression procedure on the storeddata (e.g., PDCP SDUs) using an existing header compression protocolcontext. Thereafter, the receiving PDCP entity may initialize the headercompression protocol context.

5. If there is no indicator to continue using the header decompressionprotocol for UM DRBs and AM DRBs, the receiving PDCP entity initializesdownlink header decompression protocol for the PDCP entity {e.g., asecond (or first) lower PDCP entity} corresponding to a second (orfirst) bearer for the target (or source) base station, and starts in aunidirectional (U) mode of a no context (NC) state.

6. Since data continues to be received from the source base station orthe target base station without interruption of transmission andreception of data, the receiving PDCP entity does not set window statevariables (e.g., RX_NEXT and RX_DELIV) to initial values for UM DRBs.

7. The receiving PDCP entity may store and prepare a new security keyand a new ciphering algorithm received from an upper layer entity (e.g.,an RRC entity), and may apply the same to the PDCP entity of the secondbearer for the target base station and to data, or may performdeciphering. In addition, the receiving PDCP entity may continue toapply the security key and ciphering algorithm, used before receivingthe new security key and ciphering algorithm from the upper layerentity, to the PDCP entity of the first bearer for the source basestation and to data, or may perform deciphering.

8. The receiving PDCP entity may store and prepare a new security keyand a new integrity protection algorithm received from an upper layerentity (e.g., an RRC entity), and may apply the same to the PDCP entityof the second bearer for the target base station and to data, or mayperform integrity protection. In addition, the receiving PDCP entity maycontinue to apply the security key and integrity protection algorithm,used before receiving the new security key and integrity protectionalgorithm from the upper layer entity, to the PDCP entity of the firstbearer for the source base station and to data, or may perform integrityprotection.

9. Upon receiving a PDCP status report, the receiving PDCP entity mayread the current state of transmitting and receiving data to and fromthe source base station or the target base station (e.g., successful orunsuccessful reception of data), and may reflect the same to thetransmission or retransmission of data of the transmitting PDCP entityfor the source base station or the target base station. For example, thereceiving PDCP entity may not perform transmission or retransmission onthe data of which the successful delivery is confirmed.

10. If a user data compression procedure (e.g., uplink data compression)is indicated to the terminal, the terminal may initialize a buffer forthe user data compression procedure for the source base station or thetarget base station. As another method, if dictionary information ispredefined, the buffer may be initialized using the dictionaryinformation. As another method, if there is no indicator to continueusing the buffer content for the user data compression procedure, theterminal may initialize the buffer. As another method, if dictionaryinformation is predefined, the buffer may be initialized using thedictionary information. In addition, the terminal may apply user datacompression configuration for the target base station, and mayinitialize the buffer.

If an upper layer entity (for example, an RRC entity) requests a5-2^(nd) embodiment of a PDCP entity procedure for a certain bearer, orif a third condition is satisfied, the transmitting PDCP entity of theterminal may perform the 5-2^(nd) embodiment and specifically, performone or more of a plurality of procedures below.

1. If the handover command message has no indicator to continue to usethe header compression protocol for each bearer of UM DRBs and AM DRBs,the terminal may initialize the header compression protocol in a PDCPentity of the second bearer (e.g., a second lower PDCP entity) for thetarget base station, and may start in a unidirectional (U) mode of aninitialized and refreshed (IR) state.

If the handover command message has an indicator to continue to use theheader compression protocol, the terminal may continue to use the headercompression protocol using the header compression protocol in a PDCPentity of the first bearer (e.g., a first lower PDCP entity) for thesource base station, or may perform a header decompression procedure.

2. The transmitting PDCP entity may apply a new security key and a newciphering algorithm, which are received from an upper layer entity(e.g., an RRC entity) and stored, to a PDCP entity of the second bearerfor the target base station and to data. In addition, the transmittingPDCP entity may release or discard the security key and cipheringalgorithm applied to the PDCP entity of the first bearer for the sourcebase station before receiving the new security key and cipheringalgorithm from the upper layer entity.

As another method, the transmitting PDCP entity may release the securitykey and ciphering algorithm applied to the PDCP entity of the firstbearer for the source base station in the uplink, and may maintain andapply the same to the data received in the downlink. As another method,the transmitting PDCP entity may discard or release the security key andciphering algorithm applied to the PDCP entity of the first bearer forthe source base station when completely releasing the connection withthe source base station (e.g., downlink data reception), and maycontinue to apply the same to the data received from or transmitted tothe source base station until the connection with the source basestation is completely released.

3. The transmitting PDCP entity may apply a new security key and a newintegrity verification algorithm, which are received from an upper layerentity (e.g., an RRC entity) and stored, to the PDCP entity of thesecond bearer for the target base station and to data. In addition, thetransmitting PDCP entity may release or discard the security key andciphering algorithm applied to the PDCP entity of the first bearer forthe source base station before receiving the new security key andintegrity verification algorithm from the upper layer entity.

As another method, the transmitting PDCP entity may release the securitykey and ciphering algorithm applied to the PDCP entity of the firstbearer for the source base station in the uplink, and may maintain andapply the same to the data received in the downlink. As another method,the transmitting PDCP entity may discard or release the security key andciphering algorithm applied to the PDCP entity of the first bearer forthe source base station when completely releasing the connection withthe source base station (e.g., downlink data reception), and maycontinue to apply the same to the data received from or transmitted tothe source base station until the connection with the source basestation is completely released.

4. The transmitting PDCP entity may transmit a PDCP status report to thesource base station or the target base station, thereby reporting thecurrent transmission/reception state of data (e.g., successful orunsuccessful reception of data), when it is necessary, configured, orindicated, or at all times.

5. As to UM DRBs, the transmitting PDCP entity regards the data (e.g.,PDCP SDUs), which has not been transmitted to a lower layer entity eventhough the PDCP serial numbers have already been assigned thereto (afterdiscarding all of the existing stored PDCP PDUs), as the data receivedfrom an upper layer (e.g., an SDAP entity or a TCP/IP entity), andtransmits data in the ascending order of COUNT values (or PDCP serialnumbers) allocated before the re-establishment of the PDCP. In addition,the terminal does not restart a data discard timer. Specifically, thetransmitting PDCP entity performs a new header compression procedure onthe data (PDCP SDUs), performs again an integrity procedure or aciphering procedure, configures a PDCP header, and transmits the same toa lower layer entity.

6. As to AM DRBs, the transmitting PDCP entity may perform (afterdiscarding all of the existing stored PDCP PDUs) a new headercompression procedure on the data, in the ascending order of COUNTvalues (or PDCP serial numbers) allocated before the re-establishment ofthe PDCP (or before the third condition is satisfied or before the RRCmessage is received), from the first data (e.g., PDCP SDUs) of which thesuccessful delivery has not been confirmed by lower layer entities(e.g., RLC entities). In addition, the transmitting PDCP entity mayperform again an integrity procedure or a ciphering procedure,configures a PDCP header, and may transmit the same to a lower layerentity, thereby performing retransmission or transmission. That is, thetransmitting PDCP entity may perform retransmission by accumulating datafrom the first data of which the successful delivery has not beenconfirmed.

As another method, the transmitting PDCP entity may performretransmission only on the data of which the successful delivery has notbeen confirmed by lower layer entities (e.g., RLC entities) whenperforming retransmission. Alternatively, the transmitting PDCP entitymay perform selective retransmission, based on a PDCP status report.More specifically, as to AM DRBs, the transmitting PDCP entity (afterdiscarding all of the PDCP PDUs stored to be transmitted to the sourcebase station through a first protocol entity previously connected to thePDCP entity) may release the lower layer entities (e.g., an RLC entityor a MAC entity), which are the first protocol entities for transmittingdata to the source base station. The transmitting PDCP entity mayperform a new header or data compression procedure from the first data(e.g., PDCP SDUs) of which the successful transmission has not beenconfirmed by lower layer entities (e.g., RLC entities), which are thefirst protocol entities, in the ascending order of COUNT values (or PDCPserial numbers) allocated before the re-establishment of the PDCP (orbefore the third condition is satisfied or before the RRC message isreceived) by applying the security key or the header compression (ordata compression) protocol context corresponding to the target basestation, may perform the integrity or ciphering procedure again, mayconfigure a PDCP header, and may transmit the PDCP header to the lowerlayer entity, which is the second protocol entity for transmitting datato the target base station, thereby performing retransmission ortransmission. That is, the transmitting PDCP entity may performretransmission by accumulating data from the first data of which thesuccessful delivery has not been confirmed.

As another method, when performing retransmission, the transmitting PDCPentity transmit only the data of which the successful delivery has notbeen confirmed by lower layer entities (e.g., RLC entities), which arefirst protocol entities for transmitting data to the source basestation, to the lower layer entity, which is the second protocol entityfor transmitting data to the target base station, thereby performingselective retransmission. As another method, the transmission orretransmission operation may be performed after releasing the lowerlayer entities (e.g., RLC entities or MAC entities), which are firstprotocol entities for transmitting data to the source base station.

7. If a user data compression procedure (e.g., uplink data compression)is indicated to the terminal, the terminal may initialize a buffer forthe user data compression procedure for the source base station or thetarget base station. As another method, if dictionary information ispredefined, the buffer may be initialized using the dictionaryinformation. In addition, the terminal may release user data compressionconfiguration for the source base station, and may release the buffer.

If an upper layer entity (for example, an RRC entity) requests a5-2^(nd) embodiment of a PDCP entity procedure for a certain bearer, orif a third condition is satisfied, the receiving PDCP entity of theterminal may perform the 5-2^(nd) embodiment and specifically, performone or more of a plurality of procedures below.

1. If there is data (e.g., PDCP PDUs) received from lower layer entitiesdue to re-establishment of lower layer entities (e.g., RLC entities),the receiving PDCP entity processes the data.

2. Since data continues to be received from the source base station orthe target base station without interruption of transmission andreception of data, a separate process on the stored data for UM DRBs isnot required.

3. Since data continues to be received from the source base station orthe target base station without interruption of transmission andreception of data, the receiving PDCP entity does not set window statevariables (e.g., RX_NEXT and RX_DELIV) to initial values for AM DRBs orUM DRBs.

4. The receiving PDCP entity may apply a new security key and a newciphering algorithm, which are received from an upper layer entity(e.g., an RRC entity) and stored, to the PDCP entity of the secondbearer for the target base station and to data. In addition, if downlinkdata is no longer received from the source base station, the receivingPDCP entity may release or discard the security key and cipheringalgorithm applied to the PDCP entity of the first bearer for the sourcebase station before receiving the new security key and cipheringalgorithm from the upper layer entity.

As another method, the receiving PDCP entity may release the securitykey and ciphering algorithm applied to the PDCP entity of the firstbearer for the source base station in the uplink, and may maintain andapply the same to the data received in the downlink. As another method,the receiving PDCP entity may discard or release the security key andciphering algorithm applied to the PDCP entity of the first bearer forthe source base station when completely releasing the connection withthe source base station (e.g., downlink data reception), and maycontinue to apply the same to the data received from or transmitted tothe source base station until the connection with the source basestation is completely released.

5. The receiving PDCP entity may apply a new security key and a newintegrity verification algorithm, which are received from an upper layerentity (e.g., an RRC entity) and stored, to the PDCP entity of thesecond bearer for the target base station and to data. In addition, ifdownlink data is no longer received from the source base station, thereceiving PDCP entity may release or discard the security key andciphering algorithm applied to the PDCP entity of the first bearer forthe source base station before receiving the new security key andintegrity verification algorithm from the upper layer entity.

As another method, the receiving PDCP entity may release the securitykey and ciphering algorithm applied to the PDCP entity of the firstbearer for the source base station in the uplink, and may maintain andapply the same to the data received in the downlink. As another method,the receiving PDCP entity may discard or release the security key andciphering algorithm applied to the PDCP entity of the first bearer forthe source base station when completely releasing the connection withthe source base station (e.g., downlink data reception), and maycontinue to apply the same to the data received from or transmitted tothe source base station until the connection with the source basestation is completely released.

6. Upon receiving a PDCP status report, the receiving PDCP entity mayread the current state of transmitting and receiving data to and fromthe source base station or the target base station (e.g., successful orunsuccessful reception of data), and may reflect the same to thetransmission or retransmission of data. For example, the receiving PDCPentity may not perform transmission or retransmission on the data ofwhich the successful delivery is confirmed.

7. If a user data compression procedure (e.g., uplink data compression)is indicated to the terminal, the terminal may initialize a buffer forthe user data compression procedure for the source base station or thetarget base station. As another method, if dictionary information ispredefined, the buffer may be initialized using the dictionaryinformation. In addition, the terminal may release user data compressionconfiguration for the source base station, and may release the buffer.

The third condition may be one of the following conditions.

-   -   When the terminal performs a procedure of random access to the        target base station through the entities of the second bearer        and receives a random access response    -   when the terminal performs a procedure of random access to the        target base station through the entities of the second bearer,        receives a random access response, and configures and transmits        a handover completion message to the target base station    -   when the terminal completes a procedure of random access to the        target base station through the entities of the second bearer        and initially transmits data using a PUCCH or PUSCH uplink        transmission resource    -   when the base station configures a separate timer for the        terminal through an RRC message and the timer expires    -   the timer may start when the terminal receives a handover        command message from the source base station, when the terminal        starts random access to the target base station (when        transmitting a preamble), when the terminal receives a random        access response from the target base station, when the terminal        transmits a handover completion message to the target base        station, or when the terminal initially transmits data using a        PUCCH or PUSCH uplink transmission resource.    -   when the terminal performs a procedure of random access to the        target base station through the entities of the second bearer,        receives a random access response, and configures and transmits        a handover completion message to the target base station and        when the successful transmission of the handover completion        message is identified by a MAC entity (HARQ ACK) or an RLC        entity (RLC ACK)    -   when the terminal performs a procedure of random access to the        target base station through the entities of the second bearer,        receives a random access response, configures and transmits a        handover completion message to the target base station, and        receives an uplink transmission resource that is initially        allocated by the target base station, or receives an initial        indication of an uplink transmission resource    -   when the terminal receives, from the target base station, an        indication {e.g., an RRC message (e.g., an “RRCReconfiguration”        message), a MAC CE, an RLC control PDU, or a PDCP control PDU}        to switch the uplink to the target base station    -   when the terminal receives, from the source base station, an        indication {e.g., an RRC message (e.g., an “RRCReconfiguration”        message), a MAC CE, an RLC control PDU, or a PDCP control PDU}        to switch the uplink to the target base station    -   when the terminal receives, from the target base station, an        indication {e.g., an RRC message (e.g., an “RRCReconfiguration”        message), a MAC CE, an RLC control PDU, or a PDCP control PDU}        to release the connection with the source base station    -   when the terminal receives, from the source base station, an        indication {e.g., an RRC message (e.g., an “RRCReconfiguration”        message), a MAC CE, an RLC control PDU, or a PDCP control PDU}        to release the connection with the source base station    -   when the terminal releases the connection with the source base        station and releases first protocol entities    -   when the terminal fails to receive downlink data from the source        base station for a predetermined period of time    -   when the terminal successfully completes a procedure of random        access to the target base station through the entities of the        second bearer, and receives a first uplink transmission resource        allocated by the target base station, or receives an indication        of a first uplink transmission resource    -   for example, more specifically, if the terminal is instructed to        perform random access to the target base station by receiving a        handover command message from the source base station, and if        the indicated random access is contention-free random access        (CFRA) {for example, if a predetermined preamble or a UE cell        identifier (e.g., C-RNTI) is allocated}        -   the terminal may determine that the random access procedure            is successfully completed when the terminal transmits a            predetermined preamble to the cell of the target base            station and receives a random access response (RAR) message.            Accordingly, when the terminal receives a first uplink            transmission resource allocated, included, or indicated in            the random access response message, the terminal may            determine that the third condition is satisfied.    -   If the terminal is instructed to perform random access to the        target base station by receiving a handover command message from        the source base station, and if the indicated random access is        contention-based random access (CBRA) {for example, if a        predetermined preamble or a UE cell identifier (e.g., C-RNTI) is        not allocated)        -   the terminal may determine that the procedure of random            access to the target base station is successfully completed            when the terminal transmits a preamble (e.g., an arbitrary            preamble) to the cell of the target base station, receives a            random access response (RAR) message, transmits message 3            (e.g., a handover completion message) using the uplink            transmission resource allocated, included, or indicated in            the random access response message, and receives a MAC CE            (contention resolution MAC CE) indicating that the            contention has been resolved from the target base station.            Accordingly, thereafter, when the terminal monitors a PDCCH            and initially receives an uplink transmission resource            through the PDCCH corresponding to the C-RNTI of the            terminal or initially receives an indication thereof, the            terminal may determine that the third condition is            satisfied. As another method, if the size of the uplink            transmission resource allocated by the random access            response message is sufficient enough for the terminal to            further transmit uplink data, as well as message 3, the            terminal may determine that an initial uplink transmission            resource has been received, thereby determining that the            third condition is satisfied.    -   In the case where a handover method that does not require a        random access procedure (RACH-less handover) is indicated by the        handover command message received by the terminal    -   if the handover command message includes an uplink transmission        resource to the target base station        -   the terminal may determine that the random access procedure            is successfully completed when the terminal transmits            message 3 (e.g., a handover completion message or an            “RRCReconfigurationComplete” message) using the uplink            transmission resource of the target base station and            receives a UE identity confirmation MAC CE from the base            station, thereby determining that the third condition is            satisfied. As another method, when the terminal receives a            first uplink transmission resource through a PDCCH            corresponding to a C-RNTI of the terminal by monitoring the            PDCCH after a random access procedure is successfully            completed, the terminal may determine that the third            condition is satisfied.    -   In the case where the handover command message does not include        an uplink transmission resource for the target base station        -   when the terminal receives an uplink transmission resource            through a PDCCH corresponding to a C-RNTI of the terminal by            monitoring the PDCCH for the target base station (or cell),            or when the terminal transmits message 3 (e.g., a handover            completion message or an “RRCReconfigurationComplete”            message) using the uplink transmission resource and receives            a UE identity confirmation MAC CE from the base station, the            terminal may determine that the random access procedure is            successfully completed, thereby determining that the third            condition is satisfied. As another method, when the terminal            receives a first uplink transmission resource through a            PDCCH corresponding to a C-RNTI of the terminal by            monitoring the PDCCH after a random access procedure is            successfully completed, the terminal may determine that the            third condition is satisfied.

If the third condition is not satisfied in the above step, the terminalmay repeat to check the third condition while continuing the existingprocedure.

The embodiments of the PDCP entity procedure proposed in the disclosuremay be extended and applied to the structure in which the upper PDCPentity performs a header compression procedure on the data transmittedin the PDCP entity structure shown in FIG. 11 using a common headercompression protocol context (e.g., an ROHC context) and in which therespective lower PDCP entities 1121 and 1122 perform cipheringprocedures on the data using different security keys.

In addition, the embodiments of the PDCP entity procedure proposed inthe disclosure may be extended and applied to the structure in which therespective lower PDCP entities 1121 and 1122 perform decipheringprocedures on the data received in the PDCP entity structure shown inFIG. 11 using different security keys and in which the upper PDCP entityperforms a header decompression procedure on the data using a commonheader compression protocol context (e.g., an ROHC context).

According to the specific embodiments of the PDCP entity proposed in thedisclosure, different procedures may be performed depending on the typeof handover indicated by the handover command message received by theterminal as follows.

-   -   If the type of handover indicated in the handover command        message received from the source base station by the terminal is        first handover (for example, a normal handover procedure)    -   the terminal may perform the first embodiment of the PDCP entity        proposed in the disclosure (for example, PDCP re-establishment).    -   If the type of handover indicated in the handover command        message received from the source base station by the terminal is        second handover (for example    -   the handover method of the fourth embodiment or the fifth        embodiment proposed in the disclosure), the terminal may perform        the third, fourth, or fifth embodiment of the PDCP entity        proposed in the disclosure (for example, PDCP entity        continuation).

In addition, in the case where the source base station in the disclosureinstructs the terminal to perform handover applying the embodimentsproposed in the disclosure, the source base station may start forwardingdata to the target base station when the following fifth condition issatisfied. The fifth condition may mean that one or more of thefollowing conditions are satisfied.

When the terminal receives an indication indicating that handover hasbeen successfully completed from the target base station

-   -   when a handover command message is transmitted to the terminal    -   when a handover command message is transmitted to the terminal        and successful delivery of the handover command message (HARQ        ACK, NACK, RLC ACK, or NACK) is confirmed    -   when the source base station receives, from the terminal, an        indication to release the connection with the source base        station {e.g., an RRC message (e.g., an “RRCReconfiguration”        message), a MAC CE, an RLC control PDU, or a PDCP control PDU}    -   when a predetermined timer is driven after transmitting a        handover command message to the terminal and then expires    -   when confirmation of successful transmission of downlink data        (HARQ ACK, NACK, RLC ACK, or NACK) is not received from the        terminal for a predetermined time

FIG. 13 illustrates the structure of a terminal to which an embodimentmay be applied.

Referring to FIG. 13 , the terminal includes a radio frequency (RF)processor 1310, a baseband processor 1320, a storage unit 1330, and acontroller 1340.

The RF processor 1310 performs a function of transmitting and receivinga signal through a radio channel, such as band conversion andamplification of a signal. That is, the RF processor 1310 up-converts abaseband signal provided from the baseband processor 1320 to an RF bandsignal to thus transmit the same through an antenna, and down-convertsan RF band signal received through the antenna to a baseband signal. Forexample, the RF processor 1310 may include a transmission filter, areception filter, an amplifier, a mixer, an oscillator, adigital-to-analog converter (DAC), an analog-to-digital converter (ADC),and the like. Although only one antenna is illustrated in FIG. 13 , theterminal may have a plurality of antennas. In addition, the RF processor1310 may include a plurality of RF chains. Further, the RF processor1310 may perform beamforming. To perform beamforming, the RF processor1310 may adjust the phases and magnitudes of signals transmitted andreceived through a plurality of antennas or antenna elements. Inaddition, the RF processor may perform MIMO, and may receive multiplelayers when performing the MIMO operation. The RF processor 1310 mayperform reception beam sweeping by appropriately setting a plurality ofantennas or antenna elements under the control of the controller, or mayadjust the direction and beam width of the reception beam such that thereception beam is coordinated with the transmission beam.

The baseband processor 1320 performs a function of conversion between abaseband signal and a bit string according to the physical layerspecification of the system. For example, in the case of datatransmission, the baseband processor 1320 encodes and modulatestransmission bit strings, thereby generating complex symbols. Inaddition, upon receiving data, the baseband processor 1320 demodulatesand decodes a baseband signal provided from the RF processor 1310 tothus recover reception bit strings. For example, in the case where anorthogonal frequency division multiplexing (OFDM) scheme is applied,when transmitting data, the baseband processor 1320 generates complexsymbols by encoding and modulating transmission bit strings, maps thecomplex symbols to subcarriers, and then configures OFDM symbols throughan inverse fast Fourier transform (IFFT) operation and cyclic prefix(CP) insertion. In addition, when receiving data, the baseband processor1320 divides the baseband signal provided from the RF processor 1310into OFDM symbol units, restores the signals mapped to the subcarriersthrough a fast Fourier transform (FFT) operation, and then restoresreception bit strings through demodulation and decoding.

The baseband processor 1320 and the RF processor 1310 transmit andreceive signals as described above. Accordingly, the baseband processor1320 and the RF processor 1310 may be referred to as a “transmitter”, a“receiver”, a “transceiver”, or a “communication unit”. Further, atleast one of the baseband processor 1320 and the RF processor 1310 mayinclude a plurality of communication modules in order to support aplurality of different radio access techniques. In addition, at leastone of the baseband processor 1320 and the RF processor 1310 may includedifferent communication modules to process signals in differentfrequency bands. For example, the different radio access techniques mayinclude an LTE network, an NR network, and the like. In addition, thedifferent frequency bands may include super-high frequency (SHF) (e.g.,2.5 GHz or 5 GHz) bands or millimeter wave (e.g., 60 GHz) bands.

The storage unit 1330 stores data such as fundamental programs,application programs, and configuration information for the operation ofthe terminal. The storage unit 1330 provides the stored data in responseto a request from the controller 1340.

The controller 1340 controls the overall operation of the terminal. Forexample, the controller 1340 transmits and receives signals through thebaseband processor 1320 and the RF processor 1310. In addition, thecontroller 1340 records and reads data in and from the storage unit1330. To this end, the controller 1340 may include at least oneprocessor. For example, the controller 1340 may include a communicationprocessor (CP) for controlling communication and an applicationprocessor (AP) for controlling upper layers such as applicationprograms. In some embodiments, the controller 1340 includes amulti-connection processor 1342.

FIG. 14 illustrates a block diagram of a TRP in a wireless communicationsystem to which an embodiment may be applied.

As shown in FIG. 14 , the base station includes an RF processor 1410, abaseband processor 1420, a backhaul transceiver 1430, a storage unit (ormemory) 1440, and a controller 1450.

The RF processor 1410 performs a function of transmitting and receivingsignals, such as band conversion and amplification of a signal, througha radio channel. That is, the RF processor 1410 up-converts a basebandsignal provided from the baseband processor 1420 to an RF band signal tothus transmit the same through an antenna, and down-converts an RF bandsignal received through the antenna to a baseband signal. For example,the RF processor 1410 may include a transmission filter, a receptionfilter, an amplifier, a mixer, an oscillator, a DAC, an ADC, and thelike. Although only one antenna is shown in the drawing, the firstaccess node may have a plurality of antennas. In addition, the RFprocessor 1410 may include a plurality of RF chains. Further, the RFprocessor 1410 may perform beamforming. To perform beamforming, the RFprocessor 1410 may adjust the phases and magnitudes of signalstransmitted and received through a plurality of antennas or antennaelements. The RF processor may perform a downlink MIMO operation bytransmitting one or more layers.

The baseband processor 1420 performs a function of conversion between abaseband signal and a bit string according to a physical layerspecification of a first radio access technique. For example, in thecase of data transmission, the baseband processor 1420 encodes andmodulates transmission bit strings, thereby generating complex symbols.In addition, upon receiving data, the baseband processor 1420demodulates and decodes a baseband signal provided from the RF processor1410 to thus recover reception bit strings. For example, in the casewhere an OFDM scheme is applied, when transmitting data, the basebandprocessor 1420 generates complex symbols by encoding and modulatingtransmission bit strings, maps the complex symbols to subcarriers, andthen configures OFDM symbols through the IFFT operation and CPinsertion. In addition, when receiving data, the baseband processor 1420divides the baseband signal provided from the RF processor 1410 intoOFDM symbol units, restores the signals mapped to the subcarriersthrough the FFT operation, and then restores reception bit stringsthrough demodulation and decoding. The baseband processor 1420 and theRF processor 1410 transmit and receive signals as described above.Accordingly, the baseband processor 1420 and the RF processor 1410 maybe referred to as a “transmitter”, a “receiver”, a “transceiver”, a“communication unit”, or a “wireless communication unit”.

The transceiver 1430 provides an interface for performing communicationwith other nodes in the network.

The storage unit 1440 stores data such as fundamental programs,application programs, and configuration information for the operation ofthe primary base station. In particular, the storage unit 1440 may storeinformation about bearers allocated to a connected terminal, ameasurement result reported from a connected terminal, and the like. Inaddition, the storage unit 1440 may store information that is acriterion for determining whether a multi-connection is provided to theterminal or is released. In addition, the storage unit 1440 provides thestored data in response to a request from the controller 1450.

The controller 1450 controls the overall operation of the primary basestation. For example, the controller 1450 transmits and receives signalsthrough the baseband processor 1420 and the RF processor 1410 or thebackhaul transceiver 1430. In addition, the controller 1450 records andreads data in and from the storage unit 1440. To this end, thecontroller 1450 may include at least one processor. In some embodiments,the controller 1450 includes a multi-connection processor 1452.

Meanwhile, the order of explaining the method of the disclosure withreference to the accompanying drawings does not necessarily correspondto the order of executing the method, and the sequence thereof may bechanged or executed in parallel.

Alternatively, only some elements may be included in the drawingsdescribing the method of the disclosure while some elements are omittedtherefrom without departing from the scope of the disclosure.

In addition, the method of the disclosure may be executed bycombinations of some or all of the contents included in the respectiveembodiments without departing from the scope of the disclosure.

Further, the information included in the message in the disclosure isintended to explain an example of the disclosure, and some informationmay be omitted therefrom, or additional information may be includedtherein.

Although the present disclosure has been described with variousembodiments, various changes and modifications may be suggested to oneskilled in the art. It is intended that the present disclosure encompasssuch changes and modifications as fall within the scope of the appendedclaims.

What is claimed is:
 1. A method performed by a terminal in acommunication system, the method comprising: receiving, from a firstbase station, a radio resource control (RRC) message indicating handoverto a second base station, the RRC message including configurationinformation on a data radio bearer (DRB) for the handover using a dualprotocol stack, wherein the DRB is associated with a radio link control(RLC)-acknowledged mode (AM) bearer or a RLC-unacknowledged mode (UM)bearer; identifying a medium access control (MAC) entity associated withthe second base station based on the RRC message; identifying a RLCentity for the DRB associated with the second base station; identifyinga packet data convergence protocol (PDCP) entity for the DRB, the PDCPentity being associated with a RLC entity associated with the first basestation and the RLC entity associated with the second base station; andperforming, at the PDCP entity, transmission of a PDCP service data unit(SDU) to the RLC entity associated with the second base station based onsuccessful completion of a random access procedure with the second basestation.
 2. The method of claim 1, wherein the performing comprises:performing, at the PDCP entity for the DRB associated with the RLC-AMbearer, transmission of all of one or more PDCP SDUs from a first PDCPSDU in an ascending order of a counter value associated with acorresponding PDCP SDU to the RLC entity associated with the second basestation, based on the successful completion of the random accessprocedure, wherein a successful delivery of the first PDCP SDU has notbeen confirmed by the RLC entity associated with the first base station,and wherein the one or more PDCP SDUs are already associated with PDCPsequence numbers (SNs).
 3. The method of claim 2, wherein the performingcomprises: performing, at the PDCP entity for the DRB associated withthe RLC-AM bearer, a header compression of the one or more PDCP SDUsbased on a robust header compression (ROHC) protocol; performing, at thePDCP entity for the DRB associated with the RLC-AM bearer, an integrityprotection and ciphering of the one or more PDCP SDUs based on securityinformation for the second base station; and submitting, at the PDCPentity for the DRB associated with the RLC-AM bearer, the one or morePDCP SDUs to the RLC entity associated with the second base station. 4.The method of claim 1, wherein the performing comprises: performing, atthe PDCP entity for the DRB associated with the RLC-UM bearer,transmission of all of one or more PDCP service data units (SDUs) in anascending order of a counter value associated with a corresponding PDCPSDU to the RLC entity associated with the second base station, based onthe successful completion of the random access procedure, wherein theone or more PDCP SDUs have been processed by the PDCP entity and havenot yet been submitted to the RLC entity associated with the first basestation.
 5. The method of claim 4, wherein the performing comprises:performing, at the PDCP entity for the DRB associated with the RLC-UMbearer, a header compression of the one or more PDCP SDUs based on arobust header compression (ROHC) protocol; performing, at the PDCPentity for the DRB associated with the RLC-UM bearer, an integrityprotection and ciphering of the one or more PDCP SDUs based on securityinformation for the second base station; and submitting, at the PDCPentity for the DRB associated with the RLC-UM bearer, the one or morePDCP SDUs to the RLC entity associated with the second base station. 6.The method of claim 1, further comprising: in case that the PDCP entityis configured to transmit a PDCP status report, transmitting, at thePDCP entity, a PDCP status report to the first base station based on thesuccessful completion of the random access procedure.
 7. The method ofclaim 1, wherein the PDCP entity is configured with separate securityinformation for the first base station and the second base station, andwherein a state variable used in the PDCP entity is not reset.
 8. Aterminal in a communication system, comprising: a transceiver; and acontroller coupled with the transceiver and configured to: receive, froma first base station, a radio resource control (RRC) message indicatinghandover to a second base station, the RRC message includingconfiguration information on a data radio bearer (DRB) for the handoverusing a dual protocol stack, wherein the DRB is associated with a radiolink control (RLC)-acknowledged mode (AM) bearer or a RLC-unacknowledgedmode (UM) bearer, identify a medium access control (MAC) entityassociated with the second base station based on the RRC message,identify a RLC entity for the DRB associated with the second basestation, identify a packet data convergence protocol (PDCP) entity forthe DRB, the PDCP entity being associated with a RLC entity associatedwith the first base station and the RLC entity associated with thesecond base station, perform, at the PDCP entity, transmission of a PDCPservice data unit (SDU) to the RLC entity associated with the secondbase station based on successful completion of a random access procedurewith the second base station.
 9. The terminal of claim 8, wherein thecontroller is configured to: perform, at the PDCP entity for the DRBassociated with the RLC-AM bearer, transmission of all of one or morePDCP SDUs from a first PDCP SDU in an ascending order of a counter valueassociated with a corresponding PDCP SDU to the RLC entity associatedwith the second base station, based on the successful completion of therandom access procedure, wherein a successful delivery of the first PDCPSDU has not been confirmed by the RLC entity associated with the firstbase station, and wherein the one or more PDCP SDUs are alreadyassociated with PDCP sequence numbers (SNs).
 10. The terminal of claim9, wherein the controller is configured to: perform, at the PDCP entityfor the DRB associated with the RLC-AM bearer, a header compression ofthe one or more PDCP SDUs based on a robust header compression (ROHC)protocol, perform, at the PDCP entity for the DRB associated with theRLC-AM bearer, an integrity protection and ciphering of the one or morePDCP SDUs based on security information for the second base station, andsubmit, at the PDCP entity for the DRB associated with the RLC-AMbearer, the one or more PDCP SDUs to the RLC entity associated with thesecond base station.
 11. The terminal of claim 8, wherein the controlleris configured to: perform, at the PDCP entity for the DRB associatedwith the RLC-UM bearer, transmission of all of one or more PDCP servicedata units (SDUs) in an ascending order of a counter value associatedwith a corresponding PDCP SDU to the RLC entity associated with thesecond base station, based on the successful completion of the randomaccess procedure, wherein the one or more PDCP SDUs have been processedby the PDCP entity and have not yet been submitted to the RLC entityassociated with the first base station.
 12. The terminal of claim 11,wherein the controller is configured to: perform, at the PDCP entity forthe DRB associated with the RLC-UM bearer, a header compression of theone or more PDCP SDUs based on a robust header compression (ROHC)protocol, perform, at the PDCP entity for the DRB associated with theRLC-UM bearer, an integrity protection and ciphering of the one or morePDCP SDUs based on security information for the second base station, andsubmit, at the PDCP entity for the DRB associated with the RLC-UMbearer, the one or more PDCP SDUs to the RLC entity associated with thesecond base station.
 13. The terminal of claim 8, wherein the controlleris further configured to: in case that the PDCP entity is configured totransmit a PDCP status report, transmit, at the PDCP entity, a PDCPstatus report to the first base station based on the successfulcompletion of the random access procedure.
 14. The terminal of claim 8,wherein the PDCP entity is configured with separate security informationfor the first base station and the second base station, and wherein astate variable used in the PDCP entity is not reset.